Semiconductor device

ABSTRACT

A semiconductor device having favorable electrical characteristics is provided. The semiconductor device includes a source electrode layer and a drain electrode layer which are electrically connected to an oxide semiconductor layer, a gate insulating film over the oxide semiconductor layer; the source electrode layer, and the drain electrode layer; and a gate electrode layer that overlaps with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer with the gate insulating film positioned therebetween. The source electrode layer and the drain electrode layer each include a first conductive layer and a second conductive layer. The first conductive layer is in contact with a top surface of the oxide semiconductor layer. The second conductive layer is in contact with a side surface of the oxide semiconductor layer. The first conductive layer and the second conductive layer are electrically connected to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice using an oxide semiconductor and a method for manufacturing thesemiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method for driving any ofthem, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a memory device, adisplay device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. The transistor is used in a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (also simply referred to as a display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention

For example, a technique for forming a transistor using zinc oxide or anIn—Ga—Zn-based oxide semiconductor as an oxide semiconductor isdisclosed (see Patent Documents 1 and 2).

In recent years, demand for integrated circuits in which semiconductorelements such as miniaturized transistors are integrated with highdensity has risen with increased performance and reductions in the sizeand weight of electronic devices.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide asemiconductor device with favorable electrical characteristics. Anotherobject of one embodiment of the present invention is to provide asemiconductor device with a high on-state current. Another object of oneembodiment of the present invention is to provide a semiconductor devicethat is suitable for miniaturization. In addition, another object is toprovide a highly integrated semiconductor device. Another object of oneembodiment of the present invention is to provide a semiconductor devicewith low power consumption. Another object of one embodiment of thepresent invention is to provide a semiconductor device with highreliability. Another object is to provide a semiconductor device whichcan retain data even when power supply is stopped. Another object is toprovide a novel semiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention relates to a transistorincluding an oxide semiconductor layer in a channel formation region. Asource electrode layer and a drain electrode layer of the transistoreach include a first conductive layer in contact with a first surface ofthe oxide semiconductor layer and a second conductive layer in contactwith a surface that is different from the first surface of the oxidesemiconductor layer.

One embodiment of the present invention is a semiconductor deviceincluding an insulating layer, an oxide semiconductor layer over theinsulating layer, a source electrode layer and a drain electrode layerelectrically connected to the oxide semiconductor layer, a gateinsulating film over the oxide semiconductor layer, the source electrodelayer, and the drain electrode layer, and a gate electrode layeroverlapping with the oxide semiconductor layer, the source electrodelayer, and the drain electrode layer with the gate insulating filmpositioned therebetween. The source electrode layer and the drainelectrode layer each include a first conductive layer and a secondconductive layer. The first conductive layer is in contact with a topsurface of the oxide semiconductor layer. The second conductive layer isin contact with a side surface of the oxide semiconductor layer. Thefirst conductive layer and the second conductive layer are electricallyconnected to each other.

Note that in this specification and the like, ordinal numbers such as“first”, “second”, and the like are used in order to avoid confusionamong components and do not limit the number.

In the above semiconductor device, a conductive layer overlapping withthe oxide semiconductor layer with the insulating layer positionedtherebetween may be foamed.

The oxide semiconductor layer may be a stack including a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer in this order from the insulating layer side.

It is preferable that the first to third oxide semiconductor layers eachinclude an In-M-Zn oxide layer (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, orHf), and that an atomic ratio of M with respect to In in each of thefirst and third oxide semiconductor layers be higher than an atomicratio of M with respect to In in the second oxide semiconductor layer.

The oxide semiconductor layer preferably includes a c-axis alignedcrystal.

Another embodiment of the present invention is a semiconductor deviceincluding an insulating layer, a stack in which a first oxidesemiconductor layer and a second oxide semiconductor layer are formed inthis order over the insulating layer, a source electrode layer and adrain electrode layer electrically connected to the stack, a third oxidesemiconductor layer in contact with the stack, the source electrodelayer and the drain electrode layer, a gate insulating film over thethird oxide semiconductor layer, a gate electrode layer overlapping withthe stack, the source electrode layer, the drain electrode layer, andthe third oxide semiconductor layer with the gate insulating filmpositioned therebetween. The source electrode layer and the drainelectrode layer each include a first conductive layer and a secondconductive layer. The first conductive layer is in contact with a topsurface of the second oxide semiconductor layer and the third oxidesemiconductor layer. The second conductive layer is in contact with thethird oxide semiconductor layer and side surfaces of the first oxidesemiconductor layer and the second oxide semiconductor layer. The firstconductive layer and the second conductive layer are electricallyconnected to each other.

It is preferable that the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer beeach an In-M-Zn oxide (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf),and that an atomic ratio of M to In in each of the first oxidesemiconductor layer and the third oxide semiconductor layer be higherthan an atomic ratio of M to In in the second oxide semiconductor layer.

Each of the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer preferablyincludes a crystal in which c-axes are aligned.

In the semiconductor devices of the above two embodiments, the firstconductive layer is preferably a single layer or a stacked layer that isformed using a material selected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn,Nd, and Sc and an alloy of any of these metal materials.

In the semiconductor devices of the above two embodiments, the secondconductive layer is preferably a single layer or a stacked layercomprising a material selected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn,Nd, and Sc and an alloy of any of these metal materials. The firstconductive layer and the second conductive layer preferably havedifferent compositions.

In the semiconductor devices of the above two embodiments, the secondconductive layer may be a single layer or a stacked layer comprising amaterial selected from titanium nitride, tantalum nitride, gold,platinum, palladium, ruthenium, and an oxynitride semiconductor.

According to one embodiment of the present invention, a semiconductordevice with favorable electrical characteristics can be provided.Alternatively, a semiconductor device that is suitable forminiaturization can be provided. Alternatively, a semiconductor devicewith a high on-state current can be provided. Alternatively, a highlyintegrated semiconductor device can be provided. Alternatively, asemiconductor device with low power consumption can be provided.Alternatively, a highly reliable semiconductor device can be provided. Asemiconductor device in which data is retained even when power supply isstopped can be provided. A novel semiconductor device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating atransistor.

FIGS. 2A and 2B each illustrate a cross section of a transistor in achannel width direction.

FIGS. 3A and 3B are cross-sectional views illustrating a transistor.

FIGS. 4A and 4B are a top view and a cross-sectional view illustrating atransistor.

FIGS. 5A and 5B each illustrate a cross section of a transistor in achannel width direction.

FIGS. 6A and 6B are each a cross-sectional view illustrating atransistor.

FIGS. 7A and 7B are a top view and a cross-sectional view illustrating atransistor.

FIGS. 8A and 8B each illustrate a cross section of a transistor in achannel width direction.

FIGS. 9A and 9B are each a cross-sectional view illustrating atransistor.

FIGS. 10A and 10B are a top view and a cross-sectional view illustratinga transistor.

FIGS. 11A and 11B are each a cross section of a transistor in a channelwidth direction.

FIGS. 12A and 12B are each a cross-sectional view illustrating atransistor.

FIGS. 13A to 13C illustrate a method for manufacturing a transistor.

FIGS. 14A to 14C illustrate a method for manufacturing a transistor.

FIGS. 15A to 15C illustrate a method for manufacturing a transistor.

FIGS. 16A to 16C illustrate a method for manufacturing a transistor.

FIGS. 17A to 17C illustrate a method for manufacturing a transistor.

FIGS. 18A to 18C illustrate a method for manufacturing a transistor.

FIGS. 19A to 19C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor.

FIGS. 20A and 20B show nanobeam electron diffraction patterns of oxidesemiconductor films and FIGS. 20C and 20D illustrate an example of atransmission electron diffraction measurement apparatus.

FIG. 21A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 21B and 21C show plan-viewTEM images.

FIGS. 22A and 22D are cross-sectional views of a semiconductor device,and FIGS. 22B and 22C are circuit diagrams thereof.

FIGS. 23A to 23C are a cross-sectional view and circuit diagrams of amemory device.

FIG. 24 illustrates a configuration example of an RF tag.

FIG. 25 illustrates a configuration example of a CPU.

FIG. 26 is a circuit diagram of a memory element.

FIG. 27A illustrates a configuration example of a display device, andFIGS. 27B and 27C are circuit diagrams of pixels.

FIG. 28 illustrates a display module.

FIGS. 29A to 29F illustrate electronic devices.

FIGS. 30A to 30F illustrate usage examples of an RF tag.

FIGS. 31A and 31B are a top view and a cross-sectional view of atransistor.

FIGS. 32A and 32B are a top view and a cross-sectional view illustratinga transistor.

FIGS. 33A and 33B are a top view and a cross-sectional view illustratinga transistor.

FIGS. 34A and 34B are a top view and a cross sectional view illustratinga transistor.

FIGS. 35A and 35B are a top view and a cross-sectional view illustratinga transistor.

FIGS. 36A and 36B are a top view and a cross-sectional view of atransistor.

FIGS. 37A and 37B are each a cross-sectional view of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and an example will be described in detail with reference tothe drawings. Note that the present invention is not limited to thefollowing description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and the scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description of the embodiments and the examplebelow. Note that in the structures of the present invention describedbelow, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, and thedescription thereof is not repeated in some cases. It is also to benoted that the same components are denoted by different hatchingpatterns in different drawings, or the hatching patterns are omitted insome cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, without being limited to a predetermined connectionrelation, for example, a connection relation shown in drawings or texts,another connection relation is disclosed in the drawings or the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

For example, in the case where X and Y are directly connected, X and Yare connected without an element that enables electrical connectionbetween X and Y (e.g., a switch, a transistor, a capacitor, an inductor,a resistor, a diode, a display element, a light-emitting element, or aload) interposed between X and Y.

For example, in the case where X and Y are electrically connected, oneor more elements that enable an electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, the switch is conducting or not conducting(is turned on or off) to determine whether current flows therethrough ornot. Alternatively, the switch has a function of selecting and changinga current path. Note that the case where X and Y are electricallyconnected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, and a buffer circuit; a signal generation circuit; amemory circuit; or a control circuit) can be connected between X and Y.For example, even when another circuit is interposed between X and Y, Xand Y are functionally connected if a signal output from X istransmitted to Y. Note that the case where X and Y are functionallyconnected includes the case where X and Y are directly connected and Xand Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or circuit provided therebetween), X and Y are functionallyconnected (i.e., the case where X and Y are functionally connected withanother circuit provided therebetween), and X and Y are directlyconnected (i.e., the case where X and Y are connected without anotherelement or circuit provided therebetween). That is, in thisspecification and the like, the explicit expression “X and Y areelectrically connected” is the same as the explicit simple expression “Xand Y are connected”.

For example, any of the following expressions can be used for the casewhere a source (or a first terminal or the like) of a transistor iselectrically connected to X through (or not through) Z1 and a drain (ora second terminal or the like) of the transistor is electricallyconnected to Y through (or not through) Z2, or the case where a source(or a first terminal or the like) of a transistor is directly connectedto one part of Z1 and another part of Z1 is directly connected to Xwhile a drain (or a second terminal or the like) of the transistor isdirectly connected to one part of Z2 and another part of Z2 is directlyconnected to Y.

Examples of the expressions include, “X Y, a source (or a first terminalor the like) of a transistor, and a drain (or a second terminal or thelike) of the transistor are electrically connected to each other, and X,the source (or the first terminal or the like) of the transistor, thedrain (or the second terminal or the like) of the transistor, and Y areelectrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit structure is defined byan expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include “a source (or a first terminalor the like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path”, “asource (or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first connection path, the firstconnection path does not include a second connection path, the secondconnection path includes a connection path through the transistor, adrain (or a second terminal or the like) of the transistor iselectrically connected to Y through at least Z2 on a third connectionpath, and the third connection path does not include the secondconnection path”, and “a source (or a first terminal or the like) of atransistor is electrically connected to X through at least Z1 on a firstelectrical path, the first electrical path does not include a secondelectrical path, the second electrical path is an electrical path fromthe source (or the first terminal or the like) of the transistor to adrain (or a second terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor is electricallyconnected to Y through at least Z2 on a third electrical path, the thirdelectrical path does not include a fourth electrical path, and thefourth electrical path is an electrical path from the drain (or thesecond terminal or the like) of the transistor to the source (or thefirst terminal or the like) of the transistor”. When the connection pathin a circuit structure is defined by an expression similar to the aboveexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope.

Note that one embodiment of the present invention is not limited tothese expressions which are just examples. Here, each of X, Y, Z1, andZ2 denotes an object (e.g., a device, an element, a circuit, a wiring,an electrode, a terminal, a conductive film, a layer, or the like).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Note that in this specification and the like, a transistor can be formedusing a variety of substrates. The type of a substrate is not limited toa certain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used, for example. Asan example of a glass substrate, a barium borosilicate glass substrate,an aluminoborosilicate glass substrate, soda lime glass substrate, andthe like can be given. For a flexible substrate, a flexible syntheticresin such as plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and polyether sulfone (PES), or acryliccan be used, for example. For an attachment film, polypropylene,polyester, polyvinyl fluoride, polyvinyl chloride, or the like can beused, for example. For a base material film, polyester, polyamide,polyimide, an inorganic vapor deposition film, paper, or the like can beused, for example. Specifically, when a transistor is formed using asemiconductor substrate, a single crystal substrate, an SOI substrate,or the like, it is possible to form a transistor with few variations incharacteristics, size, shape, or the like and with high current supplycapability and a small size. By forming a circuit with the use of such atransistor, power consumption of the circuit can be reduced or thecircuit can be highly integrated.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate and the transistor. The separation layer can be used when partor the whole of a semiconductor device formed over the separation layeris separated from the substrate and transferred onto another substrate.In such a case, the transistor can be transferred to a substrate havinglow heat resistance or a flexible substrate as well. For the aboveseparation layer, a stack including inorganic films, which are atungsten film and a silicon oxide film, or an organic resin film ofpolyimide or the like formed over a substrate can be used, for example.

In other words, a transistor may be formed using one substrate, and thentransferred to another substrate. Examples of a substrate to which atransistor is transferred include, in addition to the above-describedsubstrates over which transistors can be formed, a paper substrate, acellophane substrate, an aramid film substrate, a polyimide filmsubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, and the like. By using such asubstrate, a transistor with excellent properties or a transistor withlow power consumption can be formed, a device with high durability canbe formed, heat resistance can be provided, or reduction in weight orthickness can be achieved.

(Embodiment 1)

In this embodiment, a semiconductor device which is one embodiment ofthe present invention is described with reference to drawings.

In a transistor of one embodiment of the present invention, silicon(including strained silicon), germanium, silicon germanium, siliconcarbide, gallium arsenide, aluminum gallium arsenide, indium phosphide,gallium nitride, an organic semiconductor, an oxide semiconductor, orthe like can be used for a channel formation region. It is particularlypreferable to use an oxide semiconductor having a wider band gap thansilicon for the channel formation region.

The oxide semiconductor preferably contains at least indium (In) or zinc(Zn), for example. More preferably, the oxide semiconductor contains anoxide represented by an In-M-Zn-based oxide (M is a metal such as Al,Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf).

In the description below, unless otherwise specified, a semiconductordevice described as an example includes an oxide semiconductor in achannel formation region.

FIGS. 1A and 1B and FIGS. 2A and 2B are a top view and cross-sectionalviews of a transistor 101 of one embodiment of the present invention.FIG. 1A is the top view. FIG. 1B illustrates a cross section in thedirection of a dashed-dotted line A1-A2 in FIG. 1A. FIG. 2A illustratesa cross section in the direction of a dashed-dotted line A3-A4 in FIG.1A. FIG. 2B illustrates a cross section in the direction of adashed-dotted line A5-A6 in FIG. 1A. In FIGS. 1A and 1B and FIGS. 2A and2B, some components are enlarged, reduced in size, or omitted for easyunderstanding. In some cases, the direction of the dashed-dotted lineA1-A2 is referred to as a channel length direction, and the direction ofthe dashed-dotted line A3-A4 is referred to as a channel widthdirection.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not limited to one valuein some cases. Therefore, in this specification, the channel length isany one of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

A channel width refers to, for example, the length of a portion where asource and a drain face each other in a region where a semiconductor (ora portion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, a channel width ofone transistor is not fixed to one value in some cases. Therefore, inthis specification, a channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is fainted.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of a semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Furthermore, in this specification, in thecase where the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

A transistor 101 includes an insulating layer 120 over a substrate 110;an oxide semiconductor layer 130 over the insulating layer 120; a sourceelectrode layer 140 and a drain electrode layer 150 electricallyconnected to the oxide semiconductor layer 130; a gate insulating film160 over the oxide semiconductor layer 130, the source electrode layer140, and the drain electrode layer 150; and a gate electrode layer 170overlapping with the oxide semiconductor layer 130, part of the sourceelectrode layer 140, and part of the drain electrode layer 150 with thegate insulating film 160 provided therebetween. An insulating layer 180formed using an oxide may be provided over the above structure. Further,an insulating layer 185 formed using an oxide may be formed over theinsulating layer 180. The insulating layer 180 and the insulating layer185 may be provided as needed and another insulating layer may befurther provided thereover.

Note that functions of a “source” and a “drain” of a transistor aresometimes replaced with each other when a transistor of oppositepolarity is used or when the direction of current flowing is changed incircuit operation, for example. Therefore, the terms “source” and“drain” can be replaced with each other in this specification.

The transistor of one embodiment of the present invention has a top-gatestructure with a channel length greater than or equal to 10 nm and lessthan or equal to 300 nm. The transistor includes a region 191 (LovS)where the gate electrode layer 170 overlaps with the source electrodelayer 140 and a region 192 (LovD) where the gate electrode layer 170overlaps with the drain electrode layer 150. To reduce parasiticcapacitance, the width of each of the regions 191 and 192 in the channellength direction is preferably greater than or equal to 3 nm and lessthan 300 nm. Note that an embodiment of the present invention is notlimited thereto. For example, a structure as illustrated in FIGS. 31Aand 31B in which the regions 191 and 192 are not provided may beemployed. Alternatively, for example, offset regions may be providedbetween the gate electrode layer 170 and the source electrode layer 140and between the gate electrode layer 170 and the drain electrode layer150 as illustrated in FIGS. 32A and 32B.

Here, for a source electrode layer and a drain electrode layer of atransistor including an oxide semiconductor layer, the conductive filmwhich is easily bonded to oxygen can be used, for example. With use ofthe conductive material, oxygen vacancy is caused in the oxidesemiconductor layer, so that part of the oxide semiconductor layer hasan n-type conductivity. The n-type region serves as a source region anda drain region, whereby favorable characteristics of the transistor canbe obtained.

In addition, the oxide semiconductor layer is preferably in contact withan oxide insulating layer. With use of the oxide insulating layer as abase insulating film, for example, a channel formation region of theoxide semiconductor layer has an n-type conductivity because of oxygenvacancies or the like is supplied with oxygen to be intrinsic (i-type),whereby electric characteristics and reliability of the transistor canbe improved.

In the conventional transistor, the source electrode layer and the drainelectrode layer are in contact with the base insulating film in a wideregion. Accordingly, there is a problem in that an oxide insulatinglayer is deprived of oxygen by the source electrode layer and the drainelectrode layer and oxygen is not sufficiently supplied to the oxidesemiconductor layer. In particular, the problem is noticeable in atransistor with a short channel length, and thus, a negative shift ofthe threshold voltage and the like are likely to occur.

Thus, an object of one embodiment of the present invention is to form atransistor in which a contact area of the source and drain electrodelayers and the base insulating film is small.

In the transistor 101 illustrated in FIGS. 1A and 1B and FIGS. 2A and2B, the source electrode layer 140 includes a first conductive layer 141and a second conductive layer 142 and the drain electrode layer 150includes a first conductive layer 151 and a second conductive layer 152,

The first conductive layers 141 and 151 are formed in contact with a topsurface of the oxide semiconductor layer 130 (on the gate electrodelayer 170 side), and the second conductive layers 142 and 152 are formedin contact with side surfaces of the oxide semiconductor layer 130.

Here, the second conductive layers 142 and 152 each preferably have aside wall-like shape so that the contact area of the second conductivelayers 142 and 152 with the gate insulating layer 120 can be as small aspossible. When the second conductive layers 142 and 152 each have a sidewall-like shape, the contact area with the insulating layer 120 can bemade extremely small. Formation of the first conductive layers 141 and151 by using a conductive film which is used as a hard mask for formingthe oxide semiconductor layer 130 is simple and preferable.

For the first conductive layers 141 and 151, for example, a single layeror a stacked layer can be formed using a material selected from Al, Cr,Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and an alloy of any of these metalmaterials.

A single layer or a stacked layer formed using the same material as thesecond conductive layers 142 and 152 can be used for the secondconductive layers 142 and 152. However, the first conductive layers 141and 151 and the second conductive layers 142 and 152 are preferablyformed using materials having different compositions. This is because,in an etch-back process to make each of the second conductive layers 142and 152 have a side wall like shape, it is preferable that a differencein etching rate be large between the first conductive layers 141 and 151and the second conductive layers 142 and 152. As long as there is a bigdifference in etching rate between the first conductive layers 141 and151 and the second conductive layers 142 and 152, the first conductivelayers 141 and 151 and the second conductive layers 142 and 152 may havethe same composition. For example, a combination of an amorphous layerand a crystalline layer having the same composition, or a combination oflayers having different crystal structures and having the samecomposition may be employed.

In the above structure, although contact areas of the second conductivelayers 142 and 152 with the insulating layer 120 are extremely small,oxygen in the insulating layer 120 can move toward the second conductivelayers 142 and 152.

To make it more difficult for oxygen in the insulating layer 120 to movetoward the second conductive layers 142 and 152, a conductive materialwhich is not easily bonded to oxygen may be used for the secondconductive layers 142 and 152. For example, as the second conductivelayers 142 and 152, a single layer or a stacked layer of a materialselected from titanium nitride, tantalum nitride, gold, platinum,palladium, ruthenium, a conductive oxide, and a conductive oxynitridecan be used.

As the conductive oxide, indium tin oxide, indium tin oxide containingsilicon, indium oxide containing zinc, zinc oxide, zinc oxide containinggallium, zinc oxide containing aluminum, tin oxide, tin oxide containingfluorine, tin oxide containing antimony, or the like can be used.

As the conductive oxynitride, a material such as a metal oxide which isused for the oxide semiconductor layer 130 and to which nitrogen isadded can be used. For example, in the case where an In—Ga—Zn oxide film(IGZO film) is used for the oxide semiconductor layer 130, an In—Ga—Znoxynitride film (IGZON film) can be used for the second conductivelayers 142 and 152.

The conductive oxide and the conductive oxynitride are also preferablein that the etching rates thereof are significantly different from theetching rates of the metal materials which can be used for the firstconductive layers 141 and 151.

With the above structure, the conductivity type of the oxidesemiconductor layer can be made to be intrinsic (i-type) without asignificant reduction in the supply of oxygen to the oxide semiconductorlayer 130 from the insulating layer 120, resulting in improvedelectrical characteristics and reliability of the transistor. In thecase where the source electrode layer 140 and the drain electrode layer150 are formed of the first conductive layer 141 alone and the firstconductive layer 151 alone, respectively, the top surface of the oxidesemiconductor layer 130 becomes the main effective channel width; thus,on-state current of the transistor is decreased. When one embodiment ofthe present invention is employed, the side surfaces of the oxidesemiconductor layer 130 are also included in the effective channelwidth, so that the on-state current can be increased.

The transistor 101 of one embodiment of the present invention mayinclude a conductive film 172 between the oxide semiconductor layer 130and the substrate 110 as illustrated in FIG. 3A. When the conductivefilm is used as a second gate. electrode (back gate) layer, the on-statecurrent can be further increased and the threshold voltage can becontrolled. In order to increase the on-state current, for example, thegate electrode layer 170 and the conductive film 172 are set to have thesame potential, and the transistor is driven as a double-gatetransistor. In this case, as shown in FIG. 3B, the gate electrode layer170 and the conductive film 172 may be connected to each other through acontact hole. Further, to control the threshold voltage, a fixedpotential, which is different from a potential of the gate electrodelayer 170, is supplied to the conductive film 172.

The transistor of one embodiment of the present invention may have astructure illustrated in FIGS. 4A and 4B and FIGS. 5A and 5B. FIG. 4A isa top view and FIG. 4B is a cross-sectional view taken alongdashed-dotted line B1-B2 in FIG. 4A. A cross section taken along adashed-dotted line B3-B4 in FIG. 4A is illustrated in FIG. 5A. A crosssection taken along dashed-dotted line B5-B6 in FIG. 4A is illustratedin FIG. 5B. In FIGS. 4A and 4B and FIGS. 5A and 5B, some components areenlarged, reduced in size, or omitted for easy understanding. In somecases, the direction of dashed-dotted line B1-B2 is referred to as achannel length direction, and the direction of dashed-dotted line B3-B4is referred to as a channel width direction.

A transistor 102 shown in FIGS. 4A and 4B, FIGS. 5A and 5B differs fromthe transistor 101 in that a first oxide semiconductor layer 131, asecond oxide semiconductor layer 132, and a third oxide semiconductorlayer 133 are formed, as the oxide semiconductor layer 130, in thisorder from the insulating layer 120 side.

Oxide semiconductor layers with different compositions, for example, canbe used as the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133.

The transistor 102 may include a conductive film 172 between the oxidesemiconductor layer 130 and the substrate 110 as shown in FIGS. 6A and6B.

The transistor of one embodiment of the present invention may have astructure illustrated in FIGS. 7A and 7B, and FIGS. 8A and 8B. FIG. 7Ais a top view. FIG. 7B illustrates a cross section in the direction of adashed-dotted line C1-C2 in FIG. 7A. FIG. 8A illustrates a cross sectionin the direction of a dashed-dotted line C3-C4 in FIG. 7A. A crosssection taken along dashed-dotted line C5-C6 in FIG. 7A corresponds toFIG. 8B. In FIGS. 7A and 7B, and FIGS. 8A and 8B, some components areenlarged, reduced in size, or omitted for easy understanding. In somecases, the direction of the dashed-dotted line C1-C2 is referred to as achannel length direction, and the direction of the dashed-dotted lineC3-C4 is referred to as a channel width direction.

Specifically, a transistor 103 in FIGS. 7A and 7B and FIGS. 8A and 8Bincludes the insulating layer 120 over the substrate 110; the stack inwhich the first oxide semiconductor layer 131 and the second oxidesemiconductor layer 132 are formed in this order over the insulatinglayer 120; the source electrode layer 140 and the drain electrode layer150 electrically connected to part of the stack; the third oxidesemiconductor layer 133 covering the stack, the source electrode layer140, and the drain electrode layer 150; and the gate insulating film 160and the gate electrode layer 170 overlapping with the stack, the sourceelectrode layer 140, the drain electrode layer 150, and the third oxidesemiconductor layer 133. In addition, the insulating layer 180 formedusing an oxide may be provided over the above structure. Furthermore,the insulating layer 185 formed using the oxide may be formed over theinsulating layer 180. Note that the insulating layer 180 and theinsulating layer 185 may be provided as needed and another insulatinglayer may be further provided thereover.

Oxide semiconductor layers with different compositions, for example, canbe used as the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133.

Note that the transistor 103 illustrated in FIGS. 7A and 7B can alsohave a structure similar to any of the structures illustrated in FIGS.31A and 31B and FIGS. 32A and 32B. Examples of this case are illustratedin FIGS. 33 and 34. In this manner, not only the transistors illustratedin FIGS. 1A and 1B and FIGS. 7A and 7B but also the transistorsillustrated in a variety of drawings can have a structure similar to anyof the structures illustrated in FIGS. 31A and 31B and FIGS. 32A and32B.

FIG. 8A is an enlarged view showing some components of the transistor103 in the cross section taken along dashed-dotted line C3-C4 in FIG.7A. In that portion, the third oxide semiconductor layer 133 is formedto cover the stack including the first oxide semiconductor layer 131 andthe second oxide semiconductor layer 132. A top surface and sidesurfaces of the second oxide semiconductor layer 132 are covered by thegate electrode layer 170 with the third oxide semiconductor layer 133and the gate insulating film 160 provided between the second oxidesemiconductor layer 132 and the gate electrode layer 170.

FIG. 8B is an enlarged view showing some components of the transistor103 in the cross section taken along dashed-dotted line C5-C6 in FIG.7A. In the portion, the third oxide semiconductor layer 133 is formed tocover the stack of the first oxide semiconductor layer 131 and thesecond oxide semiconductor layer 132 with the drain electrode layer 150interposed therebetween.

Note that the transistor 103 has a structure in which the gateinsulating film 160 and the third oxide semiconductor layer 133 coverthe source electrode layer 140 and the drain electrode layer 150 in alarge area, whereby a gate leak current can be further reduced.

The transistor 103 may include a conductive film 172 between the oxidesemiconductor layer 130 and the substrate 110 as shown in FIGS. 9A and9B.

The transistor of one embodiment of the present invention may have astructure illustrated in FIGS. 10A and 10B, and FIGS. 11A and 11B. FIG.10A is the top view. A cross section taken along dashed-dotted lineD1-D2 in FIG. 10A corresponds to FIG. 10B. FIG. 11A illustrates a crosssection taken along dashed-dotted line D3-D4 in FIG. 10A. FIG. 11Billustrates a cross section taken along dashed-dotted line D5-D6 in FIG.10A. In FIGS. 10A and 10B, and FIGS. 11A and 11B, some components areenlarged, reduced in size, or omitted for easy understanding. In somecases, the direction of the dashed-dotted line D1-D2 is referred to as achannel length direction, and the direction of the dashed-dotted lineD3-D4 is referred to as a channel width direction.

A transistor 104 illustrated in FIGS. 10A and 10B and FIGS. 11A and 11Bhas the same structure as the transistor 103 except that top surfaceshapes of the gate insulating film 160 and the third oxide semiconductorlayer 133 are the same as or similar to that of the gate electrode layer170.

Since the number of lithography processes for the transistor 104 can besmaller than that of lithography processes for the transistor 103, thetransistor 104 can be manufactured more easily.

Note that the transistor 104 illustrated in FIGS. 10A and 10B can have astructure similar to any of the structures illustrated in FIGS. 31A and31B and FIGS. 32A and 32B. FIGS. 35A and 35B and FIGS. 36A and 36B showexamples of this case.

The transistor 104 may include the conductive film 172 between the oxidesemiconductor layer 130 and the substrate 110 as shown in FIGS. 12A and12B.

In each of the above structures of the transistors 101 to 104, the gateelectrode layer 170 electrically surrounds the oxide semiconductor layer130 in the channel width direction. Such a transistor structure isreferred to as a surrounded channel (S-channel) structure. In each ofthe structures of the transistors 102 to 104, selecting appropriatematerials for the three layers forming the oxide semiconductor layer 130allows current to flow in the whole of the second oxide semiconductorlayer 132. Since current flows in the second oxide semiconductor layer132 in an inner part of the oxide semiconductor layer 130, the currentis hardly influenced by interface scattering, leading to a high on-statecurrent. Note that increasing the thickness of the second oxidesemiconductor layer 132 can increase the on-state current. The thicknessof the second oxide semiconductor layer 132 may be, for example; 100 nmto 200 nm.

The transistor 101 illustrated in FIGS. 32A and 32B and the transistor104 illustrated in FIGS. 36A and 36B can, be a self-aligned transistorin which resistance of offset regions are reduced as illustrated inFIGS. 37A and 37B.

N-type low-resistance regions 143 and 153 can be formed by addition ofan impurity with the use of the gate electrode layer 170 as a mask. As amethod for adding the impurity, an ion implantation method, an iondoping method, a plasma immersion ion implantation method, or the likecan be used.

As the impurity for improving the conductivity of the oxidesemiconductor layer 130, for example, one or more selected from thefollowing can be used: phosphorus (P), arsenic (As), antimony (Sb),boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (CO, titanium (Ti), zinc (Zn),and carbon (C).

A semiconductor device using a transistor with any of the abovestructures can have favorable electrical characteristics.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 2)

In this embodiment, components of the transistors described inEmbodiment 1 are described in detail.

The substrate 110 is not limited to a simple supporting substrate, andmay be a substrate where another device such as a transistor is formed.In that case, one of the gate electrode layer 170, the source electrodelayer 140, and the drain electrode layer 150 of the transistor may beelectrically connected to the above device.

The insulating layer 120 can have a function of supplying oxygen to theoxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from the substrate 110. For this reason, theinsulating layer 120 is preferably an insulating film containing oxygenand further preferably, the insulating layer 120 is an insulating filmcontaining oxygen in which the oxygen content is higher than that in thestoichiometric composition. For example, the insulating layer 120 is afilm of which the amount of released oxygen when converted into oxygenatoms is 1.0×10¹⁹ atoms/cm³ or more in thermal desorption spectroscopy(TDS) analysis. Note that the temperature of the film surface in the TDSanalysis is preferably higher than or equal to 100° C. and lower than orequal to 700° C., or higher than or equal to 100° C. and lower than orequal to 500° C. In the case where the substrate 110 is provided withanother device as described above, the insulating layer 120 also has afunction as an interlayer insulating film. In that case, the insulatinglayer 120 is preferably subjected to planarization treatment such aschemical mechanical polishing (CMP) treatment so as to have a flatsurface.

In this embodiment, detailed description is given mainly on the casewhere the oxide semiconductor layer 130 has a three-layer structure;however, there is no limitation on the number of stacked layers. In thecase where the oxide semiconductor layer 130 is a single layer as in thetransistor 101, a layer corresponding to the second oxide semiconductorlayer 132 described in this embodiment is used. In the case where theoxide semiconductor layer 130 has a two-layer structure, for example, astructure of the oxide semiconductor layer 130 in the transistor 102 orthe transistor 103 without the third oxide semiconductor layer 133 isemployed. In such a case, the second oxide semiconductor layer 132 andthe first oxide semiconductor layer 131 can be interchanged. In the casewhere the oxide semiconductor layer 130 has a stacked-layer structure offour or more layers, for example, a structure in which another oxidesemiconductor layer is stacked over the three-layer stack described inthis embodiment or a structure in which another oxide semiconductorlayer is inserted in any one of the interfaces in the three-layer stackcan be employed.

For the second oxide semiconductor layer 132, for example, an oxidesemiconductor whose electron affinity (an energy difference between avacuum level and the conduction band minimum) is higher than those ofthe first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 is used. The electron affinity can be obtainedby subtracting an energy difference between a bottom of a conductionband and a top of a valence band (what is called an energy gap) from anenergy difference between the vacuum level and the top of the valenceband (what is called an ionization potential).

The first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 each contain one or more kinds of metal elementscontained in the second oxide semiconductor layer 132. For example, thefirst oxide semiconductor layer 131 and the third oxide semiconductorlayer 133 are preferably formed using an oxide semiconductor whoseconduction band minimum is closer to a vacuum level than that of thesecond oxide semiconductor layer 132 by 0.05 eV or more, 0.07 eV ormore, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less,0.5 eV or less, or 0.4 eV or less.

In such a structure, when an electric field is applied to the gateelectrode layer 170, a channel is formed in the second oxidesemiconductor layer 132 whose conduction band minimum is the lowest inthe oxide semiconductor layer 130.

Further, since the first oxide semiconductor layer 131 contains one ormore kinds of metal elements contained in the second oxide semiconductorlayer 132, an interface state is unlikely to be formed, at the interfacebetween the second oxide semiconductor layer 132 and the first oxidesemiconductor layer 131, compared with the interface between the secondoxide semiconductor layer 132 and the insulating layer 120 on theassumption that the second oxide semiconductor layer 132 is in contactwith the insulating layer 120. The interface state sometimes forms achannel; therefore, the threshold voltage of the transistor is changedin some cases. Thus, with the first oxide semiconductor layer 131,fluctuation in electrical characteristics of the transistors, such as athreshold voltage, can be reduced. Further, the reliability of thetransistor can be improved.

Furthermore, since the third oxide semiconductor layer 133 contains oneor more metal elements contained in the second oxide semiconductor layer132, scattering of carriers is less likely to occur at the interface ofthe second oxide semiconductor layer 132 with the third oxidesemiconductor layer 133 than at the interface with the gate insulatingfilm 160 on the assumption that the second oxide semiconductor layer 132is in contact with the gate insulating film 160. Therefore, with thethird oxide semiconductor layer 133, the field-effect mobility of thetransistor can be increased.

For the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133, for example, a material containing Al, Ti, Ga,Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that usedfor the second oxide semiconductor layer 132 can be used. Specifically,an atomic ratio of any of the above metal elements in the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 is1.5 times or more, preferably 2 times or more, further preferably 3times or more as much as that in the second oxide semiconductor layer132. Any of the above metal elements is strongly bonded to oxygen andthus has a function of suppressing generation of an oxygen vacancy in anoxide semiconductor layer. That is, an oxygen vacancy is less likely tobe generated in the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 than in the second oxide semiconductorlayer 132.

Note that when each of the first oxide semiconductor layer 131, thesecond oxide semiconductor layer 132, and the third oxide semiconductorlayer 133 is an In-M-Zn oxide containing at least indium, zinc, and M (Mis a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and thefirst oxide semiconductor layer 131 has an atomic ratio of In to M andZn which is x₁:y₁:z₁, the second oxide semiconductor layer 132 has anatomic ratio of In to M and Zn which is x₂:y₂:z₂, and the third oxidesemiconductor layer 133 has an atomic ratio of In to M and Zn which isx₃:y₃:z₃, each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Eachof y₁/x₁ and y₃/x₃ is one and a half times or more as large as y₂/x₂,preferably twice or more as large as y₂/x₂, more preferably three timesor more as large as y₂/x₂. At this time, when y₂ is greater than orequal to x₂ in the second oxide semiconductor layer 132, a transistorcan have stable electrical characteristics. However, when y₂ is 3 timesor more as large as x₂, the field-effect mobility of the transistor isreduced; accordingly, y₂ is preferably smaller than 3 times x₂.

Further, in the case where Zn and O are not taken into consideration,the proportion of In and the proportion of M in each of the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 arepreferably less than 50 atomic % and greater than or equal to 50 atomic%, respectively, and further preferably less than 25 atomic % andgreater than or equal to 75 atomic %, respectively. Further, in the casewhere Zn and O are not taken into consideration, the proportion of Inand the proportion of M in the second oxide semiconductor layer 132 arepreferably greater than or equal to 25 atomic % and less than 75 atomic%, respectively, further preferably greater than or equal to 34 atomic %and less than 66 atomic %, respectively.

The thicknesses of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 are each greater than or equal to 3 nm andless than or equal to 100 nm, preferably greater than or equal to 3 nmand less than or equal to 50 nm. The thickness of the second oxidesemiconductor layer 132 is greater than or equal to 3 nm and less thanor equal to 200 nm, preferably greater than or equal to 10 nm and lessthan or equal to 150 nm, further preferably greater than or equal to 20nm and less than or equal to 100 nm. In addition, the second oxidesemiconductor layer 132 is preferably thicker than the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133.

Note that stable electrical characteristics can be effectively impartedto a transistor in which an oxide semiconductor layer serves as achannel by reducing the concentration of impurities in the oxidesemiconductor layer to make the oxide semiconductor layer intrinsic orsubstantially intrinsic. The term “substantially intrinsic” refers tothe state where an oxide semiconductor layer has a carrier density lowerthan 1×10¹⁷ /cm³, preferably lower than 1×10¹⁵ /cm³, further preferablylower than 1×10¹³ /cm³.

In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon,and a metal element other than main components of the oxidesemiconductor layer are impurities. For example, hydrogen and nitrogenform donor levels to increase the carrier density. In addition, siliconin the oxide semiconductor layer forms an impurity level. The impuritylevel serves as a trap and might cause deterioration of electricalcharacteristics of the transistor. Accordingly, in the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133 and at interfaces between theselayers, the impurity concentration is preferably reduced.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, in secondary ion mass spectrometry (SIMS), forexample, the concentration of silicon at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,further preferably lower than 1×10¹⁸ atoms/cm³. Further, theconcentration of hydrogen at a certain depth of the oxide semiconductorlayer or in a region of the oxide semiconductor layer is lower than orequal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³,still further preferably lower than or equal to 5×10¹⁸ atoms/cm³.Further, the concentration of nitrogen at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³,still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. In order not to lower the crystallinity ofthe oxide semiconductor layer, for example, the concentration of siliconat a certain depth of the oxide semiconductor layer or in a region ofthe oxide semiconductor layer may be lower than 1×10¹⁹ atoms/cm³,preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than1×10¹⁸ atoms/cm³. Further, the concentration of carbon at a certaindepth of the oxide semiconductor layer or in a region of the oxidesemiconductor layer may be lower than 1×10¹⁹ atoms/cm³, preferably lowerthan 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³, forexample.

A transistor in which a highly purified oxide semiconductor film is usedfor a channel formation region as described above has extremely lowoff-state current. In the case where the voltage between a source and adrain is set to about 0.1 V, 5 V, or 10 V, for example, the off-statecurrent standardized on the channel width of the transistor can be aslow as several yoctoamperes per micrometer to several zeptoamperes permicrometer.

Note that as the gate insulating film of the transistor, an insulatingfilm containing silicon is used in many cases; thus, it is preferablethat, as in the transistor of one embodiment of the present invention, aregion of the oxide semiconductor layer, which serves as a channel, benot in contact with the gate insulating film for the above-describedreason. In the case where a channel is formed at the interface betweenthe gate insulating film and the oxide semiconductor layer, scatteringof carriers occurs at the interface, whereby the field-effect mobilityof the transistor is reduced in some cases. Also from the view of theabove, it is preferable that the region of the oxide semiconductorlayer, which serves as a channel, be separated from the gate insulatingfilm.

Accordingly, with the oxide semiconductor layer 130 having astacked-layer structure including the first oxide semiconductor layer131, the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133, a channel can be formed in the second oxidesemiconductor layer 132; thus, the transistor can have a highfield-effect mobility and stable electrical characteristics.

In the band structures of the first oxide semiconductor layer 131, thesecond oxide semiconductor layer 132, and the third oxide semiconductorlayer 133, the energy of the conduction band minimum continuouslychanges. This can be understood also from the fact that the compositionsof the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133 areclose to one another and oxygen is easily diffused among the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133. Thus, the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133 have a continuous physicalproperty although they have different compositions and form a stack. Inthe drawings in this specification, interfaces between the oxidesemiconductor layers of the stack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the samemain components are stacked is formed to have not only a simplestacked-layer structure of the layers but also a continuous energy band(here, in particular, a well structure having a U shape in which theconduction band minimums are continuous (U-shaped well)). In otherwords, the stacked-layer structure is formed such that there exists noimpurity that forms a defect level such as a trap center or arecombination center at each interface. If impurities are mixed betweenthe stacked oxide semiconductor layers, the continuity of the energyband is lost and carriers disappear by a trap or recombination.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:6:4, or 1:9:6 can be used for the firstoxide semiconductor layer 131 and the third oxide semiconductor layer133 and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:1:1, 5:5:6, or 3:1:2 can be used for the second oxide semiconductorlayer 132. Alternatively, it is possible to use an In—Ga—Zn oxide whoseatomic ratio of In to Ga and Zn is 1:6:4 or 1:9:6 for the first oxidesemiconductor layer 131 and an In—Ga—Zn oxide whose atomic ratio of Into Ga and Zn is 1:3:2, 1:3:3, or 1:3:4 for the third oxide semiconductorlayer 133, for example. Note that the atomic ratio of each of the firstoxide semiconductor layer 131, the second oxide semiconductor layer 132,and the third oxide semiconductor layer 133 varies within a range of±40% of the above atomic ratio as an error.

The second oxide semiconductor layer 132 of the oxide semiconductorlayer 130 serves as a well, so that a channel is formed in the secondoxide semiconductor layer 132 in a transistor including the oxidesemiconductor layer 130. Note that since the energies of the bottoms ofthe conduction bands are changed continuously, the oxide semiconductorlayer 130 can also be referred to as a U-shaped well. Further, a channelformed to have such a structure can also be referred to as a buriedchannel.

Note that trap levels due to impurities or defects might be formed inthe vicinity of the interface between an insulating film such as asilicon oxide film and each of the first oxide semiconductor layer 131and the third oxide semiconductor layer 133. The second oxidesemiconductor layer 132 can be distanced away from the trap levels owingto existence of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133.

However, when the energy differences between the conduction band minimumof the second oxide semiconductor layer 132 and the conduction bandminimum of each of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 are small, an electron in the second oxidesemiconductor layer 132 might reach the trap level by passing over theenergy differences. When the electron to be a negative charge is trappedin the trap state, the threshold voltage of the transistor is shifted inthe positive direction.

Thus, to reduce fluctuations in the threshold voltage of the transistor,energy differences of at least certain values between the conductionband minimum of the second oxide semiconductor layer 132 and theconduction band minimum of each of the first oxide semiconductor layer131 and the third oxide semiconductor layer 133 is necessary. Each ofthe energy differences is preferably greater than or equal to 0.1 eV,further preferably greater than or equal to 0.15 eV.

The first oxide semiconductor layer 131, the second oxide semiconductorlayer 132, and the third oxide semiconductor layer 133 preferablycontain crystal parts. In particular, when a crystal in which c-axes arealigned is used, the transistor can have stable electricalcharacteristics. Moreover, crystals with c-axis alignment are resistantto bending; therefore, using such crystals can improve the reliabilityof a semiconductor device using a flexible substrate.

As the first conductive layers 141 and 151 in the source electrode layer140 and the drain electrode layer 150, for example, a single layer or astacked layer formed using a material selected from Al, Cr, Cu, Ta, Ti,Mo, W, Ni, Mn, Nd, and Sc and an alloy of any of these metal materialscan be used. Typically, it is preferable to use Ti, which isparticularly easily bonded to oxygen, or W, which has a high meltingpoint and thus allows subsequent process temperatures to be relativelyhigh. It is also possible to use a stack of any of the above materialsand Cu or an alloy such as Cu—Mn, which has low resistance.

The above materials are capable of extracting oxygen from an oxidesemiconductor film. Therefore, in a region of the oxide semiconductorlayer that is in contact with any of the above materials, oxygen isreleased from the oxide semiconductor film and an oxygen vacancy isformed. Hydrogen slightly contained in the film and the oxygen vacancyare bonded to each other, whereby the region is markedly changed to ann-type region. Accordingly, the n-type regions can serve as a source ora drain region of the transistor.

For the second conductive layers 142 and 152, for example, a singlelayer or stacked layers comprising a material which can be used for thefirst conductive layers 141 and 151, and a material selected fromtitanium nitride, tantalum nitride, gold, platinum, palladium,ruthenium, a conductive oxide, and a conductive oxynitride can be used.Note that the material used for the first conductive layers 141 and 151is preferably different from that used for the second conductive layers142 and 152.

The gate insulating film 160 can be formed using an insulating filmcontaining one or more of aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Thegate insulating film 160 may be a stack of any of the above materials.The gate insulating film 160 may contain lanthanum (La), nitrogen, orzirconium (Zr) as an impurity.

An example of a stacked-layer structure of the gate insulating film 160will be described. The gate insulating film 160 includes, for example,oxygen, nitrogen, silicon, or hafnium. Specifically, the gate insulatingfilm 160 preferably includes hafnium oxide and silicon oxide, or hafniumoxide and silicon oxynitride.

Hafnium oxide has higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, by using hafnium oxide, a physicalthickness can be made larger than an equivalent oxide thickness; thus,even in the case where the equivalent oxide thickness is less than orequal to 10 nm or less than or equal to 5 nm, leakage current due totunnel current can be small. That is, it is possible to provide atransistor with a small off-state current. Moreover, hafnium oxide witha crystalline structure has higher dielectric constant than hafniumoxide with an amorphous structure. Therefore, it is preferable to usehafnium oxide with a crystalline structure in order to provide atransistor with a small off-state current. Examples of the crystallinestructure include a monoclinic crystal structure and a cubic crystalstructure. Note that one embodiment of the present invention is notlimited to the above examples.

A surface over which the hafnium oxide having a crystal structure isformed might have interface states due to defects. The interface statesmight function as trap centers. Therefore, in the case where the hafniumoxide is provided close to the channel region of the transistor, theelectrical characteristics of the transistor might deteriorate owing tothe interface states. In order to reduce the adverse effect of theinterface state, in some cases, it is preferable to separate the channelregion of the transistor and the hafnium oxide from each other byproviding another film therebetween. The film has a buffer function. Thefilm having a buffer function may be included in the gate insulatingfilm 160 or included in the oxide semiconductor film. That is, the filmhaving a buffer function can be formed using silicon oxide, siliconoxynitride, an oxide semiconductor, or the like. Note that the filmhaving a buffer function is formed using, for example, a semiconductoror an insulator having a larger energy gap than a semiconductor to bethe channel region. Alternatively, the film having a buffer function isformed using, for example, a semiconductor or an insulator having lowerelectron affinity than a semiconductor to be the channel region. Furtheralternatively, the film having a buffer function is formed using, forexample, a semiconductor or an insulator having higher ionization energythan a semiconductor to be the channel region.

Meanwhile, charge is trapped by the interface states (trap centers) ofthe hafnium oxide having a crystal structure, whereby the thresholdvoltage of the transistor may be controlled. In order to make theelectric charge exist stably, for example, an insulator having a largerenergy gap than hafnium oxide may be provided between the channel regionand the hafnium oxide. Alternatively, a semiconductor or an insulatorhaving smaller electron affinity than the hafnium oxide is provided. Thefilm having a buffer function may be formed using a semiconductor or aninsulator having higher ionization energy than hafnium oxide. Use ofsuch a semiconductor or an insulator inhibits discharge of the chargetrapped by the interface states, so that the charge can be retained fora long time.

Examples of such an insulator include silicon oxide and siliconoxynitride. In order to make the interface state in the gate insulatingfilm 160 trap an electric charge, an electron may be transferred fromthe oxide semiconductor layer 130 toward the gate electrode layer 170.As a specific example, the potential of the gate electrode layer 170 iskept higher than the potential of the source electrode or the drainelectrode under high temperature conditions (e.g., a temperature higherthan or equal to 125° C. and lower than or equal to 450° C., typicallyhigher than or equal to 150° C. and lower than or equal to 300° C.) forone second or longer, typically for one minute or longer.

The threshold voltage of a transistor in which a predetermined amount ofelectrons are trapped in interface states in the gate insulating film160 or the like shifts in the positive direction. The amount ofelectrons to be trapped (the amount of change in threshold voltage) canbe controlled by adjusting a voltage of the gate electrode layer 170 ortime in which the voltage is applied. Note that a location in which anelectric charge is trapped is not necessarily limited to the inside ofthe gate insulating film 160 as long as an electric charge can betrapped therein. A stacked-layer film having a similar structure may beused for another insulating layer.

For the gate electrode layer 170, for example, a conductive film formedusing Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Mn, Nd, Sc, Ta, W, orthe like can be used. It is also possible to use an alloy or aconductive nitride of any of these materials. It is also possible to usea stack of a plurality of materials selected from these materials,alloys of these materials, and conductive nitrides of these materials.Typically, tungsten, a stack of tungsten and titanium nitride, a stackof tungsten and tantalum nitride, or the like can be used. It is alsopossible to use Cu or an alloy such as Cu—Mn, which has low resistance,or a stack of any of the above materials and Cu or an alloy such asCu—Mn.

An aluminum oxide film is preferably included in the insulating layer180 over the gate insulating film 160 and the gate electrode layer 170.The aluminum oxide film has a high blocking effect of preventingpenetration of both oxygen and impurities such as hydrogen and moisture.Accordingly, during and after the manufacturing process of thetransistor, the aluminum oxide film can suitably function as aprotective film that has effects of preventing entry of impurities suchas hydrogen and moisture, which cause variations in the electricalcharacteristics of the transistor, into the oxide semiconductor layer130, preventing release of oxygen, which is a main component of theoxide semiconductor layer 130, from the oxide semiconductor layer, andpreventing unnecessary release of oxygen from the insulating layer 120.Further, oxygen contained in the aluminum oxide film can be diffused inthe oxide semiconductor layer.

Further, the insulating layer 185 is preferably formed over theinsulating layer 180. The insulating layer 185 can be formed using aninsulating film containing one or more of magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theoxide insulating layer may be a stack of any of the above materials.

Here, like the insulating layer 120, the insulating layer 185 preferablycontains oxygen more than that in the stoichiometric composition. Oxygenreleased from the insulating layer 185 can be diffused into the channelformation region in the oxide semiconductor layer 130 through the gateinsulating film 160, so that oxygen vacancies faulted in the channelformation region can be filled with the oxygen. In this manner, stableelectrical characteristics of the transistor can be achieved.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of electrical characteristics of the transistor. Adecrease in channel width causes a reduction in on-state current.

In the transistor 103 and the transistor 104 of embodiments of thepresent invention, for example, the third oxide semiconductor layer 133is formed so as to cover the second oxide semiconductor layer 132 wherea channel is formed, and the channel formation layer and the gateinsulating film are not in contact with each other, as described above.Accordingly, scattering of carriers at the interface between the channelformation layer and the gate insulating film can be reduced and theon-state current of the transistor can be increased.

In the transistor of one embodiment of the present invention, asdescribed above, the gate electrode layer 170 is formed to electricallysurround the oxide semiconductor layer 130 in the channel widthdirection; accordingly, a gate electric field is applied to the oxidesemiconductor layer 130 in the side surface direction in addition to theperpendicular direction. In other words, a gate electric field isapplied to the entire channel formation layer and an effective channelwidth is increased, leading to a further increase in the on-statecurrent.

In the transistor 103 and the transistor 104 of embodiments of thepresent invention, the second oxide semiconductor layer 132 where achannel is fanned is formed over the first oxide semiconductor layer131, so that an interface state is less likely to be formed. Inaddition, impurities do not enter the second oxide semiconductor layer132 from above and below because the second oxide semiconductor layer132 is a layer positioned at the middle of a three-layer structure.Therefore, the transistor can achieve not only the increase in theon-state current of the transistor but also stabilization of thethreshold voltage and a reduction in the S value (subthreshold value).Thus, Icut (current when gate voltage VG is 0 V) can be reduced andpower consumption can be reduced. Further, the threshold voltage of thetransistor becomes stable; thus, long-term reliability of thesemiconductor device can be improved. In addition, the transistor of oneembodiment of the present invention is suitable for a highly integratedsemiconductor device because deterioration of electrical characteristicsdue to miniaturization is reduced.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

(Embodiment 3)

In this embodiment, methods for manufacturing the transistors 101, 102,103 and 104 in Embodiment 1 are described.

First, the method for manufacturing the transistor 102 is described withreference to FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C. Inaddition, the method for manufacturing the transistor 101, which differsfrom the transistor 102 only in the structure of the oxide semiconductorlayer 130, is described. In each of FIGS. 13A to 13C, FIGS. 14A to 14C,and FIGS. 15A to 15C. A cross section of the transistor in the channellength direction is shown on the left side, and a cross section of thetransistor in the channel width direction is shown on the right side.The cross-sectional views in the channel width direction are enlargedviews; therefore, components on the left side and those on the rightside differ in apparent thickness.

For the substrate 110, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like can be used. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like, acompound semiconductor substrate made of silicon germanium or the like,a silicon-on-insulator (SOI) substrate, or the like may be used. Stillalternatively, any of these substrates further provided with asemiconductor element may be used.

The insulating layer 120 can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film ofaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or thelike; a nitride insulating film of silicon nitride, silicon nitrideoxide, aluminum nitride, aluminum nitride oxide, or the like; or a filmin which any of the above materials are mixed. Alternatively, a stackincluding any of the above materials may be used, and at least an upperlayer of the insulating layer 120 which is in contact with the oxidesemiconductor layer 130 is preferably formed using a material containingexcess oxygen that can serve as a supply source of oxygen to the oxidesemiconductor layer 130.

Oxygen may be added to the insulating layer 120 by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, or the like. Adding oxygen enables the insulating layer 120 tosupply oxygen much easily to the oxide semiconductor layer 130.

In the case where a surface of the substrate 110 is made of an insulatorand there is no influence of impurity diffusion to the oxidesemiconductor layer 130 to be formed later, the insulating layer 120 isnot necessarily provided.

Then, a first oxide semiconductor film 131 a to be the first oxidesemiconductor layer 131, a second oxide semiconductor film 132 a to bethe second oxide semiconductor layer 132, and a third oxidesemiconductor film 133 a to be the third oxide semiconductor layer 133are formed over the insulating layer 120, the source electrode layer140, and the drain electrode layer 150 by a sputtering method, a CVDmethod, an MBE method, or the like.

Note that in the case where the transistor 101 in FIGS. 1A and 1B isformed, a single film of the second oxide semiconductor film 132 a isprovided.

In the case where the oxide semiconductor layer 130 has a stacked-layerstructure, oxide semiconductor films are preferably formed successivelywithout exposure to the air with the use of a multi-chamber depositionapparatus (e.g., a sputtering apparatus) including a load lock chamber.It is preferable that each chamber of the sputtering apparatus be ableto be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa)by an adsorption vacuum evacuation pump such as a cryopump and that thechamber be able to heat a substrate over which a film is to be depositedto 100° C. or higher, preferably 500° C. or higher, so that water andthe like acting as impurities of an oxide semiconductor are removed asmuch as possible. Alternatively, a combination of a turbo molecular pumpand a cold trap is preferably used to prevent back-flow of a gascontaining a carbon component, moisture, or the like from an exhaustsystem into the chamber. Alternatively, a combination of a turbomolecular pump and a cryopump may be used as an exhaust system.

Not only high evacuation in a chamber but also high purity of asputtering gas is necessary to obtain a high-purity intrinsic oxidesemiconductor. When a highly purified gas having a dew point of −40° C.or lower, preferably −80° C. or lower, more preferably −100° C. or loweris used as an oxygen gas or an argon gas used as a sputtering gas,moisture or the like can be prevented from entering an oxidesemiconductor as much as possible.

For the first oxide semiconductor film 131 a, the second oxidesemiconductor film 132 a, and the third oxide semiconductor film 133 a,any of the materials described in Embodiment 1 can be used. For example,an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4,1:3:3, or 1:3:2 can be used for the first oxide semiconductor film 131a, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1,3:1:2, or 5:5:6 can be used for the second oxide semiconductor film 132a, and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6,1:3:4, 1:3:3, or 1:3:2 can be used for the third oxide semiconductorfilm 133 a. Note that the atomic ratio of each of the first oxidesemiconductor film 131 a, the second oxide semiconductor film 132 a, andthe third oxide semiconductor film 133 a may vary within a range of ±20%of the above atomic ratio as an error. In the case where a sputteringmethod is used for deposition, the above material can be used as atarget.

An oxide semiconductor that can be used for each of the first oxidesemiconductor film 131 a, the second oxide semiconductor film 132 a, andthe third oxide semiconductor film 133 a preferably contains at leastindium (In) or zinc (Zn). Alternatively, both In and Zn are preferablycontained. In order to reduce fluctuations in electrical characteristicsof the transistors including the oxide semiconductor, the oxidesemiconductor preferably contains a stabilizer in addition to In and Zn.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given. As another stabilizer,lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), or lutetium (Lu) can be given.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, gallium oxide, zinc oxide, an In—Znoxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, anIn—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide,an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Znoxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, anIn—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide,an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Znoxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide,an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

Note that here, for example, an “In—Ga—Zn oxide” means an oxidecontaining In, Ga, and Zn as its main components. The In—Ga—Zn oxide maycontain another metal element in addition to In, Ga, and Zn. Further, inthis specification, a film formed using an In—Ga—Zn oxide is alsoreferred to as an IGZO film.

A material represented by InMO₃(ZnO)_(m) (m>0 is satisfied, and in isnot an integer) may be used. Note that M represents one or more metalelements selected from Ga, Y, Zr, La, Ce, and Nd. Alternatively, amaterial represented by In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may beused.

Note that as described in Embodiment 1 in detail, materials are selectedso that the first oxide semiconductor film 131 a and the third oxidesemiconductor film 133 a each have an electron affinity lower than thatof the second oxide semiconductor film 132 a.

Note that the oxide semiconductor film is preferably formed by asputtering method. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used.

The indium content in the second oxide semiconductor film 132 a ispreferably higher than those in the first and third oxide semiconductorfilms 131 a and 133 a. In an oxide semiconductor, the s orbital of heavymetal mainly contributes to carrier transfer, and when the proportion ofIn in the oxide semiconductor is increased, overlap of the s orbitals islikely to be increased. Therefore, an oxide having a composition inwhich the proportion of In is higher than that of Ga has higher mobilitythan an oxide having a composition in which the proportion of In isequal to or lower than that of Ga. Thus, with the use of an oxide havinga high indium content for the second oxide semiconductor layer 132, atransistor having high mobility can be achieved.

First heat treatment may be performed after the third oxidesemiconductor film 133 a is formed. The first heat treatment may beperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 300° C. andlower than or equal to 500° C., in an inert gas atmosphere, anatmosphere containing an oxidizing gas at 10 ppm or more, or a reducedpressure state. Alternatively, the first heat treatment may be performedin such a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, in order tocompensate desorbed oxygen. The first heat treatment can increase thecrystallinity of the first to third oxide semiconductor films 131 a to133 a and remove impurities such as water and hydrogen from theinsulating layer 120 and the first to third oxide semiconductor films131 a to 133 a. Note that the first heat treatment may be performedafter etching for formation of the first to third oxide semiconductorfilms 131 a to 133 a, which is described later.

Next, a conductive layer 145 is formed over the third oxidesemiconductor film 133 a (see FIG. 13A). The conductive layer 145 canbe, for example, formed by the following method.

First, a first conductive film is formed over the third oxidesemiconductor film 133 a. As the first conductive film, a single layeror a stacked layer can be formed using a material selected from Al, Cr,Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloys of any of these metalmaterials.

Next, a negative resist film is formed over the first conductive filmand the resist film is exposed to light by electron beam exposure,liquid immersion exposure, or EUV exposure and developed, so that afirst resist mask is formed. An organic coating film is preferablyformed as an adherence agent between the first conductive film and theresist film. Alternatively, the first resist mask may be formed bynanoimprint lithography.

Then, the first conductive film is selectively etched using the firstresist mask and the first resist mask is subjected to aching; thus, theconductive layer 145 is formed.

Next, the third oxide semiconductor film 133 a, the second oxidesemiconductor film 132 a, and the first oxide semiconductor film 131 aare selectively etched using the conductive layer 145 as a hard mask;thus, the oxide semiconductor layer 130 that is a stack including thefirst to third oxide semiconductor layers 131 to 133 is formed (see FIG.13B).

Next, a second conductive film 146 a is formed to cover the oxidesemiconductor layer 130 and the conductive layer 145. (see FIG. 13C).For the second conductive film 146 a, for example, a single layer orstacked layers comprising a material which can be used for the firstconductive film, and a material selected from titanium nitride, tantalumnitride, gold, platinum, palladium, ruthenium, a conductive oxide, and aconductive oxynitride can be used.

Then, etch back is performed to form a conductive layer 146 on sidesurfaces of the oxide semiconductor layer 130 and the conductive layer145 (see FIG. 14A).

Then, a second resist mask 155 is formed over portions to be a sourceregion and a drain region (see FIG. 14B). Then, a part of the conductivelayer 145 and a part of the conductive layer 146 formed in contact withthe oxide semiconductor layer 130 are etched, whereby the sourceelectrode layer 140 including the first conductive layer 141 and thesecond conductive layer 142, and the drain electrode layer 150 includingthe first conductive layer 151 and the second conductive layer 152 areformed (see FIG. 14C). Note that for clarification of explanation, ineach of the right parts of FIG. 14B and FIG. 14C, a cross section in achannel formation region (top) and a cross section in the drain region(bottom) are illustrated.

Next, the gate insulating film 160 is formed over the oxidesemiconductor layer 130, the source electrode layer 140, and the drainelectrode layer 150 (see FIG. 15A). The gate insulating film 160 can beformed using aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like. The gateinsulating film 160 may be a stack including any of the above materials.The gate insulating film 160 can be formed by a sputtering method, a CVDmethod, an MBE method, or the like.

Then, a third conductive film to be the gate electrode layer 170 isformed over the gate insulating film 160. For the third conductive film,a single layer, a stack, or an alloy of any of Al, Ti, Cr, Co, Ni, Cu,Y, Zr, Mo, Ru, Ag, Mn, Nd, Sc, Ta, and W can be used. The secondconductive film can be formed by a sputtering method, a CVD method, orthe like. The second conductive film may be formed using a conductivefilm containing nitrogen or a stack including the conductive film and aconductive film containing nitrogen.

After that, a third resist mask is formed over the third conductivefilm, and the third conductive film is selectively etched using thethird resist mask to form the gate electrode layer 170 (see FIG. 15B).

Then, the insulating layer 180 and the insulating layer 185 are formedover the gate insulating film 160 and the gate electrode layer 170 (seeFIG. 15C). The insulating layer 180 and the insulating layer 185 caneach be formed using a material and a method which are similar to thoseof the insulating layer 120. Note that it is particularly preferable touse aluminum oxide for the insulating layer 180.

Oxygen may be added to the insulating layer 180 and/or the insulatinglayer 185 by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or the like. Adding oxygen enablesthe insulating layer 180 and/or the insulating layer 185 to supplyoxygen much easily to the oxide semiconductor layer 130.

After that, second heat treatment may be performed. The second heattreatment can be performed in a condition similar to that of the firstheat treatment. By the second heat treatment, excess oxygen is easilyreleased from the insulating layer 120, the insulating layer 180, andthe insulating layer 185, so that oxygen vacancies in the oxidesemiconductor layer 130 can be reduced.

Through the above steps, the transistor 102 shown in FIGS. 4A and 4B canbe manufactured. In addition, as described above, when a single layer isused as the oxide semiconductor layer 130, the transistor 101 shown inFIGS. 1A and 1B can be manufactured.

Next, the method for manufacturing the transistor 103 shown in FIGS. 7Aand 7B is described. Note that detailed description of steps similar tothose for manufacturing the transistor 101 and the transistor 102described above is omitted.

The insulating layer 120 is formed over the substrate 110, and the firstoxide semiconductor film 131 a to be the first oxide semiconductor layer131 and the second oxide semiconductor film 132 a to be the second oxidesemiconductor layer 132 are formed over the insulating layer 120 by asputtering method, a CVD method, an MBE method, or the like.

After that, the first conductive film is formed over the second oxidesemiconductor film 132 a, and the conductive layer 145 is formed by themethod described above. Then, the first oxide semiconductor film 131 aand the second oxide semiconductor film 132 a are selectively etchedusing the conductive layer 145 as a hard mask, whereby a stack includingthe first oxide semiconductor layer 131, the second oxide semiconductorlayer 132, and the conductive layer 145 is formed (see FIG. 16A).

Next, the second conductive film 146 a is formed to cover the stack.Then, etch back is performed, so that the conductive layer 146 is formedin contact with side surfaces of the stack.

Then, a part of the conductive layer 145 and a part of the conductivelayer 146 are etched, whereby the source electrode layer 140 includingthe first conductive layer 141 and the second conductive layer 142 andthe drain electrode layer 150 including the first conductive layer 151and the second conductive layer 152 are formed (see FIG. 16B). Note thatfor clarification of explanation, in the right part of FIG. 16B, a crosssection in a channel formation region (top) and a cross section in adrain region (bottom) are illustrated.

Next, the third oxide semiconductor film 133 a to be the third oxidesemiconductor layer 133 is formed over the stack including the secondoxide semiconductor layer 132 and the first oxide semiconductor layer131, the source electrode layer 140, and the drain electrode layer 150.Next, the gate insulating film 160 is formed over the third oxidesemiconductor film 133 a.

Next, a third resist mask 156 is formed over the gate insulating film160 to cover the stack including the first oxide semiconductor layer 131and the second oxide semiconductor layer 132, the source electrode layer140, and the drain electrode layer 150 (see FIG. 16C).

Next, a part of the gate insulating film 160 and a part of the thirdoxide semiconductor film 133 a are etched using the third resist mask156, so that the third oxide semiconductor layer is formed (see FIG.17A).

Then, a third conductive film 170 a to be the gate electrode layer 170is formed over the gate insulating film 160 and is selectively etched toform the gate electrode layer 170 (see FIG. 17B).

Then, the insulating layer 180 and the insulating layer 185 are formedover the gate insulating film 160 and the gate electrode layer 170 (seeFIG. 17C).

Through the above steps, the transistor 103 shown in FIGS. 7A and 7B canbe manufactured.

Next, a method for manufacturing the transistor 104 illustrated in FIGS.10A and 10B is described. Note that detailed description of stepssimilar to those for manufacturing the transistors 101 to 103 describedabove is omitted.

First, the steps up to the step illustrated in FIG. 16B for forming thetransistor 103 are performed.

Next, the third oxide semiconductor film 133 a to be the third oxidesemiconductor layer 133 is formed over the stack including the firstoxide semiconductor layer 131 and the second oxide semiconductor layer132, the source electrode layer 140, and the drain electrode layer 150.Then, the gate insulating film 160 and the third conductive film 170 aare formed over the third oxide semiconductor film 133 a.

Next, a fourth resist mask 157 is formed over the third conductive film170 a (see FIG. 18A). Then, the third conductive film 170 a isselectively etched using the third resist mask 157 to form the gateelectrode layer 170.

Then, the gate insulating film 160 is selectively etched using the gateelectrode layer 170 as a mask.

After that, the third oxide semiconductor film 133 a is etched using thegate electrode layer 170 or the gate insulating film 160 as a mask toform the third oxide semiconductor layer 133 (see FIG. 18B).

The third conductive film 170 a, the gate insulating film 160, and thethird oxide semiconductor film 133 a may be etched individually orsuccessively. Further, either dry etching or wet etching may be used asthe etching method, and an appropriate etching method may be selectedindividually.

Next, the insulating layer 180 and the insulating layer 185 are formedover the source electrode layer 140, the drain electrode layer 150, andthe gate electrode layer 170 (see FIG. 18C).

Through the above steps, the transistor 104 shown in FIGS. 10A and 10Bcan be formed.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films which are described in thisembodiment typically can be formed by a sputtering method or a plasmaCVD method, such films may be formed by another method, e.g., a thermalCVD method. A metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be employed as an example ofa thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—ZnO_(X) (X>0)is formed, trimethylindium, trimethylgallium, and dimethylzinc can beused. For example, in the case where an In—Ga—Zn—O film is formed,trimethylindium, trimethylgallium, and dimethylzinc are used. Note thatthe chemical formula of trimethylindium is In(CH₃)₃. The chemicalformula of trimethylgallium is Ga(CH₃)₃. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing a liquid containing a solvent and a hafnium precursorcompound (a hafnium alkoxide solution, typicallytetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemicalformula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples ofanother material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, e.g., H₂Oas an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, an oxide semiconductor film, e.g., an In—Ga—ZnO_(X) (X>0)film is formed using a deposition apparatus employing ALD, an In(CH₃)₃gas and an O₃ gas are sequentially introduced plural times to form anIn—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at a time toform a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are introducedat a time to foam a ZnO layer. Note that the order of these layers isnot limited to this example. A mixed compound layer such as an In—Ga—Olayer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixingthese gases. Note that although an H₂O gas which is obtained by bubblingwith an inert gas such as Ar may be used instead of and O₃ gas, it ispreferable to use and O₃ gas, which does not contain H. Further, insteadof an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may beused.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 4)

In this embodiment, an oxide semiconductor film that can be used for atransistor of one embodiment of the present invention is described.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflecting a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged parallel to the formation surfaceor the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 19A is a cross-sectional TEM image of a CAAC-OS film. FIG. 19B is across-sectional TEM image obtained by enlarging the image of FIG. 19A.In FIG. 19B, atomic arrangement is highlighted for easy understanding.

FIG. 19C is Fourier transform images of regions each surrounded by acircle (the diameter is about 4 nm) between A and O and between O and A′in FIG. 19A. C-axis alignment can be observed in each region in FIG.19C. The c-axis direction between A and O is different from that betweenO and A′, which indicates that a grain in the region between A and O isdifferent from that between O and A′. In addition, the angle of thec-axis between A and O continuously and gradually changes, for example,14.3°, 16.6°, and 26.4°. Similarly, between O and A′, the angle of thec-axis continuously changes from −18.3°, −17.6°, to −15.9°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) having alignment are shown. For example, when electrondiffraction with an electron beam having a diameter of, for example, 1nm or more and 30 nm or less (such electron diffraction is also referredto as nanobeam electron diffraction) is performed on the top surface ofthe CAAC-OS film, the spots are observed (see FIG. 20A).

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are irregularlyoriented between crystal parts, the c-axes are aligned in a directionparallel to a normal vector of a formation surface or a normal vector ofa top surface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appears at around 31° and a peak of 2θ does not appear ataround 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Electric charge trapped by the carrier traps in the oxide semiconductorfilm takes a long time to be released, and might behave like fixedelectric charge. Thus, the transistor which includes the oxidesemiconductor film having high impurity concentration and a high densityof defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small. Next, a microcrystallineoxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a crystal grain boundary cannot be found clearly in the nc-OS filmin some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak which showsa crystal plane does not appear. Furthermore, a halo pattern is shown inan electron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than the diameter of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter close to, orsmaller than the diameter of a crystal part. Further, in a nanobeamelectron diffraction pattern of the nc-OS film, regions with highluminance in a circular (ring) pattern are shown in some cases. Further,in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of circumferentially distributed spots are observed (see FIG.20B).

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film

Note that an oxide semiconductor film may be a stacked film includingtwo or more kinds of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In the case where the oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 20C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 10, an optical system12 below the electron gun chamber 10, a sample chamber 14 below theoptical system 12, an optical system 16 below the sample chamber 14, anobservation chamber 20 below the optical system 16, a camera 18installed in the observation chamber 20, and a film chamber 22 below theobservation chamber 20. The camera 18 is provided to face toward theinside of the observation chamber 20. Note that the film chamber 22 isnot necessarily provided.

FIG. 20D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 20C. In thetransmission electron diffraction measurement apparatus, a substance 28which is positioned in the sample chamber 14 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 10 through the optical system 12. Electrons passing through thesubstance 28 enter a fluorescent plate 32 provided in the observationchamber 20 through the optical system 16. On the fluorescent plate 32, apattern corresponding to the intensity of entered electron appears,which allows measurement of a transmission electron diffraction pattern.

The camera 18 is installed so as to face the fluorescent plate 32 andcan take a picture of a pattern appearing in the fluorescent plate 32.An angle formed by a straight line which passes through the center of alens of the camera 18 and the center of the fluorescent plate 32 and anupper surface of the fluorescent plate 32 is, for example, 15° or moreand 80° or less, 30° or more and 75° or less, or 45° or more and 70° orless. As the angle is reduced, distortion of the transmission electrondiffraction pattern taken by the camera 18 becomes larger. Note that ifthe angle is obtained in advance, the distortion of an obtainedtransmission electron diffraction pattern can be corrected. Note thatthe film chamber 22 may be provided with the camera 18. For example, thecamera 18 may be set in the film chamber 22 so as to be opposite to theincident direction of electrons 24 enter. In this case, a transmissionelectron diffraction pattern with less distortion can be taken from therear surface of the fluorescent plate 32.

A holder for fixing the substance 28 that is a sample is provided in thesample chamber 14. The holder transmits electrons passing through thesubstance 28. The holder may have, for example, a function of moving thesubstance 28 in the direction of the X, Y, and Z axes. The movementfunction of the holder may have an accuracy of moving the substance inthe range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to 100nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range is preferablydetermined to be an optimal range for the structure of the substance 28.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above will be described.

For example, changes in the structure of a substance can be observed bychanging (or by scanning) the irradiation position of the electrons 24that are a nanobeam in the substance as illustrated in FIG. 20D. At thistime, when the substance 28 is a CAAC-OS film, a diffraction patternshown in FIG. 20A can be observed. When the substance 28 is an nc-OSfilm, a diffraction pattern shown in FIG. 20B can be observed.

Even when the substance 28 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether or not a CAAC-OS film is favorable can bedetermined by the proportion of a region where a diffraction pattern ofa CAAC-OS film is observed in a predetermined area (also referred to asproportion of CAAC). In the case of a high quality CAAC-OS film, forexample, the proportion of CAAC is higher than or equal to 50%,preferably higher than or equal to 80%, further preferably higher thanor equal to 90%, still further preferably higher than or equal to 95%.Note that the proportion of a region where a diffraction patterndifferent from that of a CAAC-OS film is observed is referred to as theproportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS subjected to heat treatment at 450° C.in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nanobeam with a probe diameter of 1 nmwas used. The above measurement was performed on six samples. Theproportion of CAAC was calculated using the average value of the sixsamples.

FIG. 21A shows the proportion of CAAC in each sample. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Further, the above results also indicate that even when the temperatureof the heat treatment is lower than 500° C., the CAAC-OS film can have ahigh proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Further, anamorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 21B and 21C are planar TEM images of the CAAC-OS film obtainedjust after the deposition and the CAAC-OS film subjected to the heattreatment at 450° C., respectively. Comparison between FIGS. 21B and 21Cshows that the CAAC-OS film subjected to the heat treatment at 450° C.has more uniform film quality. That is, the heat treatment at a hightemperature improves the film quality of the CAAC-OS film

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 5)

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention is described with reference todrawings.

[Cross-Sectional Structure]

FIG. 22A is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. The semiconductor deviceillustrated in FIG. 22A includes a transistor 2200 containing a firstsemiconductor material in a lower portion and a transistor 2100containing a second semiconductor material in an upper portion. In FIG.22A, an example is described in which the transistor described in theabove embodiment as an example is used as the transistor 2100 containingthe second semiconductor material. A cross-sectional view of thetransistors in a channel length direction is on the left side of adashed-dotted line, and a cross-sectional view of the transistors in achannel width direction is on the right side of the dashed-dotted line.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor using a material other than an oxidesemiconductor, such as single crystal silicon, can operate at high speedeasily. On the other hand, a transistor including an oxide semiconductorhas a low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIG. 22A illustrates a structure in which the transistor 2100 isprovided over the transistor 2200 with an insulating film 2201 and aninsulating film 2207 provided therebetween. A plurality of wirings 2202are provided between the transistor 2200 and the transistor 2100.Furthermore, wirings and electrodes provided over and under theinsulating films are electrically connected to each other through aplurality of plugs 2203 embedded in the insulating films An insulatingfilm 2204 covering the transistor 2100, wirings 2205 and 2209 over theinsulating film 2204, and wirings 2206 and 2210 formed by processing aconductive film that is also used in a source electrode layer and adrain electrode layer of the transistor 2100 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in a lower portion, hydrogen in aninsulating film provided in the vicinity of the semiconductor film ofthe transistor 2200 terminates dangling bonds of silicon; accordingly,the reliability of the transistor 2200 can be improved. Meanwhile, inthe case where an oxide semiconductor is used for the transistor 2100provided in an upper portion, hydrogen in an insulating film provided inthe vicinity of the semiconductor film of the transistor 2100 becomes afactor of generating carriers in the oxide semiconductor; thus, thereliability of the transistor 2100 might be decreased. Therefore, in thecase where the transistor 2100 using an oxide semiconductor is providedover the transistor 2200 using a silicon-based semiconductor material,it is particularly effective that the insulating film 2207 having afunction of preventing diffusion of hydrogen is provided between thetransistors 2100 and 2200. The insulating film 2207 makes hydrogenremain in the lower portion, thereby improving the reliability of thetransistor 2200. In addition, since the insulating film 2207 suppressesdiffusion of hydrogen from the lower portion to the upper portion, thereliability of the transistor 2100 also can be improved.

The insulating film 2207 can be, for example, formed using aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film 2208 (corresponding to the insulating layer180 in the transistors 101 to 103) having a function of preventingdiffusion of hydrogen is preferably formed over the transistor 2100 tocover the transistor 2100 including an oxide semiconductor film. For theblocking film 2208, a material that is similar to that of the insulatingfilm 2207 can be used, and in particular, an aluminum oxide film ispreferably used. The aluminum oxide film has a high shielding (blocking)effect of preventing penetration of both oxygen and impurities such ashydrogen and moisture. Thus, by using the aluminum oxide film as theblocking film 2208 covering the transistor 2100, release of oxygen fromthe oxide semiconductor layer included in the transistor 2100 can beprevented and entry of water and hydrogen into the oxide semiconductorfilm can be prevented.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. Examples of a cross-sectional view in this case areillustrated in FIG. 22D. An insulating film 2212 is provided over thesemiconductor substrate 2211. The semiconductor substrate 2211 includesa projecting portion with a thin tip (also referred to a fin). Note thatan insulating film may be provided over the protruding portion. Theinsulating film functions as a mask for preventing the semiconductorsubstrate 2211 from being etched when the protruding portion is formed.Alternatively, the protruding portion may not have the thin tip; aprotruding portion with a cuboid-like protruding portion and aprotruding portion with a thick tip are permitted, for example. A gateinsulating film 2214 is provided over the protruding portion of thesemiconductor substrate 2211, and a gate electrode 2213 is provided overthe gate insulating film 2214. Source and drain regions 2215 are formedin the semiconductor substrate 2211. Note that here is shown an examplein which the semiconductor substrate 2211 includes the projectionportion; however, a semiconductor device of one embodiment of thepresent invention is not limited thereto. For example, a semiconductorregion having a protruding portion may be formed by processing an SOIsubstrate.

[Circuit Configuration Example]

In the above structure, electrodes of the transistor 2100 and thetransistor 2200 can be connected in a variety of ways; thus, a varietyof circuits can be formed. Examples of circuit configurations which canbe achieved by using a semiconductor device of one embodiment of thepresent invention are shown below.

[CMOS Circuit]

A circuit diagram in FIG. 22B shows a configuration of a “CMOS circuit”in which the p-channel transistor 2200 and the n-channel transistor 2100are connected to each other in series and in which gates of them areconnected to each other.

[Analog Switch]

A circuit diagram in FIG. 22C shows a configuration in which a sourceand a drain of the transistor 2100 are connected to a source and a drainof the transistor 2200. With such a configuration, the transistors canfunction as a so-called analog switch.

[Memory Device Example]

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 23A to 23C.

The semiconductor device illustrated in FIG. 23A includes a transistor3200 including a first semiconductor material, a transistor 3300including a second semiconductor material, and a capacitor 3400. Notethat as the transistor 3300, any of the transistors described in theabove embodiments can be used.

FIG. 23B is a cross-sectional view of the semiconductor deviceillustrated in FIG. 23A. The semiconductor device in the cross-sectionalview has a structure in which the transistor 3300 is provided with aback gate; however, a structure without a back gate may be employed.

The transistor 3300 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 3300 is low, stored data can beretained for a long period owing to such a transistor. In other words,power consumption can be sufficiently reduced because a semiconductormemory device in which refresh operation is unnecessary or the frequencyof refresh operation is extremely low can be provided.

In FIG. 23A, a first wiring 3001 is electrically connected to a sourceelectrode of the transistor 3200. A second wiring 3002 is electricallyconnected to a drain electrode of the transistor 3200. A third wiring3003 is electrically connected to one of the source electrode and thedrain electrode of the transistor 3300. A fourth wiring 3004 iselectrically connected to the gate electrode of the transistor 3300. Thegate electrode of the transistor 3200 is electrically connected to theother of the source electrode and the drain electrode of the transistor3300 and the one electrode of the capacitor 3400. A fifth wiring 3005 iselectrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 23A utilizes a characteristic in whichthe potential of the gate electrode of the transistor 3200 can be held,and thus enables writing, storing, and reading of data as follows.

Writing and holding of data will be described. First, the potential ofthe fourth wiring 3004 is set to a potential at which the transistor3300 is turned on, so that the transistor 3300 is turned on.Accordingly, the potential of the third wiring 3003 is supplied to thegate electrode of the transistor 3200 and the capacitor 3400. That is, apredetermined charge is supplied to the gate of the transistor 3200(writing). Here, charge for supplying either of two different potentiallevels (hereinafter referred to as low-level charge and high-levelcharge) is given. After that, the potential of the fourth wiring 3004 isset to a potential at which the transistor 3300 is turned off, so thatthe transistor 3300 is turned off. Thus, the charge supplied to the gateof the transistor 3200 is retained (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the gate of the transistor 3200 is retained for a long time.

Next, reading of data will be described. An appropriate potential (areading potential) is supplied to the fifth wiring 3005 while apredetermined potential (a constant potential) is supplied to the firstwiring 3001, whereby the potential of the second wiring 3002 variesdepending on the amount of charge retained in the gate of the transistor3200. This is because in general, when the transistor 3200 is ann-channel transistor, an apparent threshold voltage V_(th) _(_) _(H) inthe case where a high-level charge is given to the gate electrode of thetransistor 3200 is lower than an apparent threshold voltage V_(th) _(_)_(L) in the case where a low-level charge is given to the gate electrodeof the transistor 3200. Here, an apparent threshold voltage refers tothe potential of the fifth wiring 3005 which is needed to turn on thetransistor 3200. Thus, the potential of the fifth wiring 3005 is set toa potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L),whereby charge supplied to the gate of the transistor 3200 can bedetermined. For example, in the case where the high-level charge issupplied in writing, when the potential of the fifth wiring 3005 is V₀(>V_(th) _(_) _(H)), the transistor 3200 is turned on. In the case wherethe low-level charge is supplied in writing, even when the potential ofthe fifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200remains off. Therefore, the data stored in the gate electrode layer canbe read by determining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. The fifth wiring 3005 inthe case where data is not read may be supplied with a potential atwhich the transistor 3200 is turned off regardless of the state of thegate, that is, a potential lower than V_(th) _(_) _(H). Alternatively,the fifth wiring 3005 may be supplied with a potential at which thetransistor 3200 is turned on regardless of the state of the gate, thatis, a potential higher than V_(th) _(_) _(L).

The semiconductor device illustrated in FIG. 23C is different from thesemiconductor device illustrated in FIG. 23A in that the transistor 3200is not provided. Also in this case, writing and holding of data can beperformed in a manner similar to the above.

Next, operation of data reading will be described. When the transistor3300 is turned on, the third wiring 3003 which is in a floating stateand the capacitor 3400 are electrically connected to each other, and thecharge is redistributed between the third wiring 3003 and the capacitor3400. As a result, the potential of the third wiring 3003 is changed.The amount of change in potential of the third wiring 3003 variesdepending on the potential of the first terminal of the capacitor 3400(or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of thefirst terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potentialof the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor materialmay be used for a driver circuit for driving a memory cell, and atransistor including the second semiconductor material may be stackedover the driver circuit as the transistor 3300.

When a transistor having a channel formation region formed using anoxide semiconductor and having extremely small off-state current isapplied to the semiconductor device in this embodiment, thesemiconductor device can store data for an extremely long period. Inother words, power consumption can be sufficiently reduced becauserefresh operation becomes unnecessary or the frequency of refreshoperation can be extremely low. Moreover, stored data can be held for along period even when power is not supplied (note that a potential ispreferably fixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating film does not arise at all. In other words, thesemiconductor device according to one embodiment of the presentinvention does not have a limit on the number of times of writing whichis a problem in a conventional nonvolatile memory, and reliabilitythereof is drastically improved. Furthermore, data is written dependingon the on state and the off state of the transistor, whereby high-speedoperation can be easily achieved.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention is clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where there are severalpossible portions to which a terminal can be connected, it is notnecessary to specify the portions to which the terminal is connected.Therefore, it might be possible to constitute one embodiment of theinvention by specifying only portions to which some of terminals of anactive element (e.g., a transistor or a diode), a passive element (e.g.,a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, oneembodiment of the present invention can be clear when a function of acircuit is specified. Further, it can be determined that one embodimentof the present invention whose function is specified is disclosed inthis specification and the like. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, it is possible to take out part of thediagram or the text and constitute an embodiment of the invention. Thus,in the case where a diagram or a text related to a certain portion isdescribed, the context taken out from part of the diagram or the text isalso disclosed as one embodiment of the invention, and one embodiment ofthe invention can be constituted. Therefore, for example, in a diagramor a text in which one or more active elements (e.g., transistors ordiodes), wirings, passive elements (e.g., capacitors or resistors),conductive layers, insulating layers, semiconductor layers, organicmaterials, inorganic materials, components, devices, operating methods,manufacturing methods, or the like are described, part of the diagram orthe text is taken out, and one embodiment of the invention can beconstituted. For example, from a circuit diagram in which N circuitelements (e.g., transistors or capacitors; N is an integer) areprovided, it is possible to constitute one embodiment of the inventionby taking out M circuit elements (e.g., transistors or capacitors; M isan integer, where M<N). As another example, it is possible to constituteone embodiment of the invention by taking out M layers (M is an integer,where M<N) from a cross-sectional view in which N layers (N is aninteger) are provided. As another example, it is possible to constituteone embodiment of the invention by taking out M elements (M is aninteger, where M<N) from a flow chart in which N elements (N is aninteger) are provided.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 6)

In this embodiment, an RF tag that includes the transistor described inthe above embodiments or the memory device described in the aboveembodiment is described with reference to FIG. 24.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. In orderthat the RFID tag is used for such application, extremely highreliability is needed.

A configuration of the RF tag will be described with reference to FIG.24. FIG. 24 is a block diagram illustrating a configuration example ofan RF tag.

As shown in FIG. 24, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, the structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the memory device described in the above embodiment can be used asthe memory circuit 810. Since the memory circuit of one embodiment ofthe present invention can retain data even when not powered, the memorycircuit can be favorably used for an RF tag. Furthermore, the memorycircuit of one embodiment of the present invention needs power (voltage)needed for data writing significantly lower than that needed in aconventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. In addition, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 7)

In this embodiment, a CPU that includes the memory device described inthe above embodiment is described.

FIG. 25 is a block diagram illustrating a configuration example of a CPUat least partly including any of the transistors described in the aboveembodiments as a component.

The CPU illustrated in FIG. 25 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198 (BusI/F), a rewritable ROM 1199, and a ROM interface 1189 (ROM I/F). Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over a separate chip. Needless to say, the CPU inFIG. 25 is just an example in which the configuration is simplified, andan actual CPU may have a variety of configurations depending on theapplication. For example, the CPU may have the following configuration:a structure including the CPU illustrated in FIG. 25 or an arithmeticcircuit is considered as one core; a plurality of cores are included;and the plurality of cores operate in parallel. The number of bits thatthe CPU can process hi an internal arithmetic circuit or in a data buscan be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 in accordance with the stateof the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 on the basis of areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 25, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of thetransistors described in the above embodiments can be used.

In the CPU illustrated in FIG. 25, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

FIG. 26 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used asthe circuit 1202. When supply of a power supply voltage to the memoryelement 1200 is stopped, a ground potential (0 V) or a potential atwhich the transistor 1209 in the circuit 1202 is turned off continues tobe input to a gate of the transistor 1209. For example, the first gateof the transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node Ml. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

Note that the transistor 1209 in FIG. 26 has a structure with a secondgate (second gate electrode; back gate). The control signal WE can beinput to the first gate and the control signal WE2 can be input to thesecond gate. The control signal WE2 is a signal having a constantpotential. As the constant potential, for example, a ground potentialGND or a potential lower than a source potential of the transistor 1209is selected. The control signal WE2 is a potential signal forcontrolling the threshold voltage of the transistor 1209, and a currentof the transistor 1209 when gate voltage VG is 0 V can be furtherreduced. The control signal WE2 may be a signal having the samepotential as that of the control signal WE. Note that as the transistor1209, a transistor without a second gate may be used.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 26illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 26, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 26, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a layer formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor in which a channel is formed in a silicon layer or asilicon substrate. Alternatively, all the transistors in the memoryelement 1200 may be a transistor in which a channel is formed in anoxide semiconductor layer. Further alternatively, in the memory element1200, a transistor in which a channel is formed in an oxidesemiconductor layer can be included besides the transistor 1209, and atransistor in which a channel is formed in a layer or the substrate 1190including a semiconductor other than an oxide semiconductor can be usedfor the rest of the transistors.

As the circuit 1201 in FIG. 25, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in th s e circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of a transistor whose channel is formed in an oxidesemiconductor layer is much lower than that of a transistor whosechannel is formed in crystalline silicon. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the memory element performs pre-charge operation with the switch1203 and the switch 1204, the time required for the circuit 1201 toretain original data again after the supply of the power supply voltageis restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Further, shortly after thesupply of the power supply voltage is restarted, the memory element canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency identification (RF-ID).

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 8)

In this embodiment, configuration examples of a display device using atransistor of one embodiment of the present invention are described.

[Structure Example]

FIG. 27A is a top view of the display device of one embodiment of thepresent invention. FIG. 27B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 27C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance with theabove embodiment. Further, the transistor can easily be an n-channeltransistor, and thus, part of a driver circuit that can be formed usingan n-channel transistor in the driver circuit is formed over the samesubstrate as the transistor of the pixel portion. With the use of thetransistor described in the above embodiment for the pixel portion orthe driver circuit in this manner, a highly reliable display device canbe provided.

FIG. 27A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extended from thesignal line driver circuit 704 are arranged and a plurality of scanlines extended from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels eachincluding a display element are provided in matrix in respective regionsin each of which the scan line and the signal line intersect with eachother. The substrate 700 of the display device is connected to a timingcontrol circuit (also referred to as a controller or a controller IC)through a connection portion such as a flexible printed circuit (FPC).

In FIG. 27A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components of a driver circuit which isprovided outside and the like are reduced, so that a reduction in costcan be achieved. Further, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofconnections of wirings would be increased, but by providing the drivercircuit over the substrate 700, the number of connections of the wiringscan be reduced. When the driver circuit is provided over the substrate700, the number of connections of the wirings can be reduced.Consequently, an improvement in reliability or yield can be achieved.

[Liquid Crystal Display Device]

FIG. 27B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A scan line 712 connected to a transistor 716 and a scan line 713connected to a transistor 717 are separated so that different gatesignals can be supplied thereto. In contrast, a signal line 714 isshared by the transistors 716 and 717. The transistor described in theabove embodiments can be used as appropriate as each of the transistors716 and 717. Thus, a highly reliable liquid crystal display device canbe provided.

The shapes of a first pixel electrode layer electrically connected tothe transistor 716 and a second pixel electrode layer electricallyconnected to the transistor 717 are described. The first pixel electrodelayer and the second pixel electrode layer are separated by a slit. Thefirst pixel electrode layer has a V shape and the second pixel electrodelayer is provided so as to surround the first pixel electrode layer.

A gate electrode of the transistor 716 is connected to the scan line712, and a gate electrode of the transistor 717 is connected to the scanline 713. When different gate signals are supplied to the scan line 712and the scan line 713, operation timings of the transistor 716 and thetransistor 717 can be varied. As a result, alignment of liquid crystalscan be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating film functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat shown in FIG. 27B. For example, a switch, a resistor, a capacitor,a transistor, a sensor, a logic circuit, or the like may be added to thepixel illustrated in FIG. 27B.

[Organic EL Display Device]

FIG. 27C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Basedon such a mechanism, such a light-emitting element is referred to as acurrent-excitation type light-emitting element.

FIG. 27C illustrates an applicable example of a pixel circuit. In thisexample, one pixel includes two n-channel transistors. Note that themetal oxide film of one embodiment of the present invention can be usedfor channel formation regions of the n-channel transistors. Further,digital time grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line provided over the same substrate.

As the switching transistor 721 and the driver transistor 722, thetransistor described in other embodiments can be used as appropriate. Inthis manner, a highly reliable organic EL display device can beprovided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 may be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted. The gate capacitance of the driver transistor 722 may be formedbetween the channel formation region and the gate electrode layer.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode layer of the driver transistor722. Note that voltage higher than or equal to voltage which is the sumof power supply line voltage and the threshold voltage Vth of the drivertransistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage greaterthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage Vth of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that shown in FIG. 27C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 27C.

In the case where the transistor shown in any of the above embodimentsis used for any of the circuits shown in FIGS. 27A to 27C, the sourceelectrode (the first electrode) is electrically connected to the lowpotential side and the drain electrode (the second electrode) iselectrically connected to the high potential side. Furthermore, thepotential of the first gate electrode may be controlled by a controlcircuit or the like and the potential described above as an example,e.g., a potential lower than the potential applied to the sourceelectrode, may be input to the second gate electrode through a wiringthat is not illustrated.

A display element, a display device, a light-emitting element, or alight-emitting device includes, for example, at least one of anelectroluminescence (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, a liquid crystal element, electronic ink, anelectrophoretic element, a grating light valve (GLV), a plasma displaypanel (PDP), a display elements using micro electro mechanical system(MEMS), a digital micromirror device (DMD), a digital micro shutter(DMS), MIRASOL (registered trademark), an interferometric modulatordisplay (IMOD) element, a MEMS shutter display element, anoptical-interference-type MEMS display element, an electrowettingelement, a piezoelectric ceramic display, and a display elementincluding a carbon nanotube. In addition, a display medium whosecontrast, luminance, reflectance, transmittance, or the like is changedby electric action or magnetic action may be included. For example, someor all of pixel electrodes are formed to contain aluminum, silver, orthe like. In such a case, a memory circuit such as an SRAM can beprovided under the reflective electrodes, leading to lower powerconsumption.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 9)

In this embodiment, a display module that can be formed using thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIG. 28.

In a display module 8000 in FIG. 28, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002. Note thatthe backlight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 to form an optical touch panel. Anelectrode for a touch sensor may be provided in each pixel of thedisplay panel 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. Note that the battery 8011 is not necessary in the casewhere a commercial power source is used.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 10)

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images), or thelike. Other examples of electronic devices that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game consoles, portabledata appliances, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines. FIGS.29A to 29F illustrate specific examples of such electronic devices.

FIG. 29A illustrates a portable game console including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game console in FIG. 29A has the two displayportions 903 and 904, the number of display portions included in aportable game console is not limited to this.

FIG. 29B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by provision of atouch panel in a display device. Alternatively, the position inputfunction can be added by provision of a photoelectric conversion elementcalled a photosensor in a pixel area of a display device.

FIG. 29C illustrates a notebook personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 29D illustrates a wrist-watch-type information terminal, whichincludes a housing 931, a display portion 932, a wristband 933, and thelike. The display portion 932 may be a touch panel.

FIG. 29E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 29F illustrates an ordinary vehicle including a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

(Embodiment 11)

In this embodiment, application examples of an RF tag of one embodimentof the present invention will be described with reference to FIGS. 30Ato 30F. The RF tag is widely used and can be provided for, for example,products such as bills, coins, securities, bearer bonds, documents(e.g., driver's licenses or resident's cards, see FIGS. 30A and 30B),vehicles (e.g., bicycles, see FIG. 30C), packaging containers (e.g.,wrapping paper or bottles, see FIG. 30D), recording media (e.g., DVD orvideo tapes), personal belongings (e.g., bags or glasses), foods,plants, animals, human bodies, clothing, household goods, medicalsupplies such as medicine and chemicals, and electronic devices (e.g.,liquid crystal display devices, EL display devices, television sets, orcellular phones), or tags on products (see FIGS. 30E and 30F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 can be reduced in size, thickness, and weight, it can befixed to a product without spoiling the design of the product. Further,bills, coins, securities, bearer bonds, documents, or the like can havean identification function by being provided with the RF tag 4000, andthe identification function can be utilized to prevent counterfeiting.Moreover, the efficiency of a system such as an inspection system can beimproved by providing the RF tag for packaging containers, recordingmedia, personal belongings, foods, clothing, household goods, electronicdevices, or the like. Vehicles can also have higher security againsttheft or the like by being provided with the RF tag of one embodiment ofthe present invention.

As described above, by using the RF tag of one embodiment of the presentinvention for each application described in this embodiment, power foroperation such as writing or reading of data can be reduced, whichresults in an increase in the maximum communication distance. Moreover,data can be held for an extremely long period even in the state wherepower is not supplied; thus, the RFID can be preferably used forapplication in which data is not frequently written or read.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

This application is based on Japanese Patent Application serial no.2013-263247 filed with Japan Patent Office on Dec. 20, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an insulatinglayer; an oxide semiconductor layer over the insulating layer; a sourceelectrode layer and a drain electrode layer on and in contact with theoxide semiconductor layer; a gate insulating film over the oxidesemiconductor layer, the source electrode layer, and the drain electrodelayer; and a gate electrode layer over the oxide semiconductor layer,the source electrode layer, and the drain electrode layer with the gateinsulating film interposed therebetween, wherein each of the sourceelectrode layer and the drain electrode layer comprises a firstconductive layer being in contact with a top surface of the oxidesemiconductor layer and a second conductive layer in contact with a sidesurface of the oxide semiconductor layer, wherein the oxidesemiconductor layer is a stack comprising a first oxide semiconductorlayer, a second oxide semiconductor layer, and a third oxidesemiconductor layer in order from the insulating layer, and wherein thesecond conductive layer is a single layer or a stacked layer comprisingat least one material selected from the group consisting of titaniumnitride, tantalum nitride, gold, platinum, palladium, ruthenium, and anoxynitride semiconductor.
 2. The semiconductor device according to claim1, wherein each of the first to third oxide semiconductor layerscomprises In, Zn, and metal, wherein the metal is selected from thegroup consisting of Al, Ti, Ga, Y, Zr, La, Ce, Nd, and Hf, and whereinan atomic ratio of the metal with respect to In in each of the first andthird oxide semiconductor layers is higher than an atomic ratio of themetal with respect to In in the second oxide semiconductor layer.
 3. Thesemiconductor device according to claim 1, wherein the first conductivelayer is a single layer or a stacked layer comprising at least onematerial selected from the group consisting of Al, Cr, Cu, Ta, Ti, Mo,W, Ni, Mn, Nd, Sc, and an alloy thereof, and wherein a composition ofthe second conductive layer is different from a composition of the firstconductive layer.
 4. A semiconductor device comprising: an insulatinglayer; a stack comprising a first oxide semiconductor layer and a secondoxide semiconductor layer in order over the insulating layer; a sourceelectrode layer and a drain electrode layer on and in contact with thestack; a third oxide semiconductor layer on and in contact with thestack, the source electrode layer, and the drain electrode layer; a gateinsulating film over the third oxide semiconductor layer; and a gateelectrode layer over the stack, the source electrode layer, the drainelectrode layer, and the third oxide semiconductor layer with the gateinsulating film interposed therebetween, wherein each of the sourceelectrode layer and the drain electrode layer comprises a firstconductive layer being in contact with a top surface of the second oxidesemiconductor layer and a second conductive layer being in contact withside surfaces of the first oxide semiconductor layer and the secondoxide semiconductor layer.
 5. The semiconductor device according toclaim 4, further comprising a conductive layer below the stack with theinsulating layer interposed therebetween.
 6. The semiconductor deviceaccording to claim 4, wherein each of the first to third oxidesemiconductor layers comprises In, Zn, and metal, wherein the metal isselected from the group consisting of Al, Ti, Ga, Y, Zr, La, Ce, Nd, andHf, and wherein an atomic ratio of the metal with respect to In in eachof the first and third oxide semiconductor layers is higher than anatomic ratio of the metal with respect to In in the second oxidesemiconductor layer.
 7. The semiconductor device according to claim 4,wherein the first to third oxide semiconductor layers each comprise ac-axis aligned crystal of oxide semiconductor.
 8. The semiconductordevice according to claim 4, wherein each of the first conductive layerand the second conductive layer is a single layer or a stacked layercomprising at least one material selected from the group consisting ofAl, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, Sc, and an alloy thereof, andwherein a composition of the second conductive layer is different from acomposition of the first conductive layer.
 9. The semiconductor deviceaccording to claim 4, wherein the second conductive layer is a singlelayer or a stacked layer comprising at least one material selected fromthe group consisting of titanium nitride, tantalum nitride, gold,platinum, palladium, ruthenium, and an oxynitride semiconductor.
 10. Asemiconductor device comprising: an insulating layer; a stack comprisinga first oxide semiconductor layer and a second oxide semiconductor layerin order over the insulating layer; a source electrode layer and a drainelectrode layer on and in contact with the stack, each of the sourceelectrode layer and the drain electrode layer comprising a firstconductive layer and a second conductive layer; a third oxidesemiconductor layer on and in contact with the stack, the sourceelectrode layer, and the drain electrode layer; a gate insulating filmover the third oxide semiconductor layer; and a gate electrode layerover the stack, the source electrode layer, the drain electrode layer,and the third oxide semiconductor layer with the gate insulating filminterposed therebetween, wherein the first conductive layer is providedbetween the second oxide semiconductor layer and the third oxidesemiconductor layer, and wherein the second conductive layer is providedon a side surface of the stack.
 11. The semiconductor device accordingto claim 10, further comprising a conductive layer below the stack withthe insulating layer interposed therebetween.
 12. The semiconductordevice according to claim 10, wherein each of the first to third oxidesemiconductor layers comprises In, Zn, and metal, wherein the metal isselected from the group consisting of Al, Ti, Ga, Y, Zr, La, Ce, Nd, andHf, and wherein an atomic ratio of the metal with respect to In in eachof the first and third oxide semiconductor layers is higher than anatomic ratio of the metal with respect to In in the second oxidesemiconductor layer.
 13. The semiconductor device according to claim 10,wherein the first to third oxide semiconductor layers each comprise ac-axis aligned crystal of oxide semiconductor.
 14. The semiconductordevice according to claim 10, wherein each of the first conductive layerand the second conductive layer is a single layer or a stacked layercomprising at least one material selected from the group consisting ofAl, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, Sc, and an alloy thereof, andwherein a composition of the second conductive layer is different from acomposition of the first conductive layer.
 15. The semiconductor deviceaccording to claim 10, wherein the second conductive layer is a singlelayer or a stacked layer comprising at least one material selected fromthe group consisting of titanium nitride, tantalum nitride, gold,platinum, palladium, ruthenium, and an oxynitride semiconductor.
 16. Asemiconductor device comprising: an insulating layer; an oxidesemiconductor layer over the insulating layer; a source electrode layerand a drain electrode layer on and in contact with the oxidesemiconductor layer; a gate insulating film over the oxide semiconductorlayer, the source electrode layer, and the drain electrode layer; and agate electrode layer over the oxide semiconductor layer, the sourceelectrode layer, and the drain electrode layer with the gate insulatingfilm interposed therebetween, wherein each of the source electrode layerand the drain electrode layer comprises a first conductive layer beingin contact with a top surface of the oxide semiconductor layer and asecond conductive layer in contact with a side surface of the oxidesemiconductor layer, and wherein the second conductive layer is a singlelayer or a stacked layer comprising at least one material selected fromthe group consisting of titanium nitride, tantalum nitride, gold,platinum, palladium, ruthenium, and an oxynitride semiconductor.
 17. Thesemiconductor device according to claim 16, further comprising aconductive layer below the oxide semiconductor layer with the insulatinglayer interposed therebetween.
 18. The semiconductor device according toclaim 16, wherein the oxide semiconductor layer comprises a c-axisaligned crystal of oxide semiconductor.
 19. The semiconductor deviceaccording to claim 16, wherein the first conductive layer is a singlelayer or a stacked layer comprising at least one material selected fromthe group consisting of Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, Sc, andan alloy thereof, and wherein a composition of the second conductivelayer is different from a composition of the first conductive layer.